Clothes Dryer Stops Heating

I've been chasing this one for a few weeks now, with no luck. Pertinent model and device information is below. I could use some advice…

Make:  Kenmore
Model: 110.96584110

After I complete a repair and/or reset of the high-temperature cut off relay, the dryer will only run for 5-10 minutes before the high-temp relay trips again. Here's what I've been able to asses:

1. Verified that I'm getting proper 240V from the electrical distribution panel and that the breaker is not being tripped (I have manually reset it for no good reason other than to reset).
2. Verified the heating element is unbroken and presents the typical small resistance on the meter. This heating element was found broken several months ago and replaced with a new element.
3. Verified that the thermal fuse has continuity.
4. Verified that the thermal cut-off fuse has continuity. This has been replaced with a new unit as part of a kit. Additionally, I have manually reset this fuse to restore continuity after other attempted repairs to see if it works.
5. Verified that the high limit thermostat has continuity. This has been replaced with a new unit as part of a kit.
6. Verified that the dryer operating thermostat (part #3387134) has the expected resistance across the large terminals (~7kOhms). Additionally, I have tested the thermostat on an electric griddle to verify the thermostat cycles with the temperature. (My limitation is I don't have an accurate reading on the temperatures at which it cycles, though I did watch it cycle at least 6-8 times.)
7. I have completely cleaned ALL duct work from the back of the dryer to the outside and verified there is strong air flow. Additionally, since I have a long run (over 27 feet with two right angles), I've installed an in-line duct booster fan to ensure consistent air flow away from the dryer (Tjernlund LB2).
8. I have completely disassembled the dryer down to the drum and cleaned out all lint from internal ductwork, checked and cleaned the drum seals, and replaced the foam seal connecting the transition duct to the blower motor. Also cleaned the blower motor barrel fan and detached and reattached all electrical connections (in some cases using a little sandpaper to get a shiny copper connector).

When I get everything reassembled, the dryer turns on and the heating element warms up to glow orange. A (meat) thermometer inserted into the duct exhaust at the back of the dryer reads up to 170 degrees before the thermostat trips and allows the heating coil to cool. This heating on/off cycle will continue for 5-10 mins until at some point the high-temp thermal fuse trips. I'm stumped as to why this is happening now when it's been operating for over twenty years.

Aside from being an elderly appliance with replaced parts — and there are no electronic control boards or other "magic black boxes" in the circuit — the control mechanism is so very simple. Yet clearly there is an overtemp condition that is tripping the fuse. I've tried everything I could think of from air handling/venting to the electrical path. Any ideas from experienced hands is much appreciated.

(My last resort will be to replace the dryer operating thermostat for about $26. If that doesn't work, we may declare total death and purchase a new one.)

UPDATE 1: I installed a new operating thermostat today and the high temp cutoff still trips. What I've figured out is that the high temp cutoff is on the heating tube — which is in line BEFORE the tumbler barrel (obviously), but the regular operating thermostat is AFTER the tumbler barrel. My thought is that the heated air wasn't getting through the tumbler to the operating thermostat. Removed all clothes and the lint screen and restarted and the dryer began cycling properly from the thermostat (whew!). BUT after several cycles, the high temp cutoff tripped again. My thought now is that the blower fan and/or motor is weak and not adequately pulling air through the tumbler system and therefore not allowing the heating tube to cool enough. The dryer drive motor ($141.63) is a standard electrical wound motor and I'm thinking it's unlikely to just "get weak" (the barrel still tumbles with a full load of laundry). So my best guess is the blower wheel? Even that doesn't make sense, so I'm wondering if the drive shaft from the motor to the blower wheel is slipping — which means there would be adequate torque to turn the tumbler barrel (via the belt drive off the motor) but the slipping shaft wouldn't deliver enough torque to the blower wheel to turn it and get sufficient air movement (CFM). I have no idea what the rated CFM should be. I'll look into the drive shaft.

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UPDATE 2: Disassembled the dryer nearly completely and removed the motor from the blower wheel, removing both of them from the dryer chassis. The motor was filthy and the drive shaft and rotor were caked with lint, dirt, and hair. I thoroughly cleaned the motor (stator and rotor) using compressed air and lubricated the drive shaft bearings with WD40. The blower wheel seemed to spin much more freely, so I was hopeful. After reassembling the whole dryer and firing it up once more, I noted clean cycling of the thermostat as desired — but only when the dryer exhaust duct was disconnected from the wall duct. My two conclusions are now: 1) the dryer motor must be weaker now than it used to be, because I didn't have this overheating issue; and 2) the air back pressure from my long duct run to vent outside is not allowing the heating element to cool sufficiently between heating cycles. This is causing the overtemp condition. The in-line booster fan has an automatic pressure sensor, measuring the pressure difference on either side of the fan. It's supposed to turn on the fan whenever it senses the dryer is blowing through the duct. Honestly, this has never seemed to work as the booster fan turns on intermittently and never for the full dryer cycle. My thought is to somehow bypass this pressure sensor and hard-wire the booster fan "on" switch to the dryer (maybe to the motor power line?) and force the booster fan to run consistently when the dryer is running. Has anyone done this before? Or any other thoughts?