Water – What would cause a gas fired, forced water boiler to shut down

I have a fairly old Peerless, natural gas fired, forced water boiler. It has an electric ignition system (White Rodgers 50E47-843 Universal HSI Ignition Module).

A few weeks ago it shut down (safety lockdown, blinking red light). After I cut and restored electricity to the system it restarted without trouble. But then, a few days later, it locked out. Any time I cycled the power to the system it restarted fine, and would be fine for a few days. I replaced the HSI igniter (glow plug) but that didn't fix it. I am unable to find an exact replacement for the flame sensor.

This morning, it locked out again. When I tried the power-toggle trick the glow plug lit up, as before, but the gas didn't kick in right away; it started after the glow plug stopped glowing.

I have two questions:

1. Any ideas as to what might be the problem?

2. If I want to replace the flame sensor, the specified flame sensor is a WR-760-403 which does not seem to exist anymore. The closest thing is the WR-760-401, which differs from the 403 in that it's a straight wire, as opposed to a bent wire in the 403. Can I used the 401? Should I bend it (in a vise, or something) to match the shape of the 403?

Update:

A few additional points:

• I've noticed this tends to happen more often overnight when the thermostat is set lower. However, it does happen at other times and higher thermostat settings, too.
• Cleaning the flame sensor did not help. I did that this AM, this afternoon when I got home it was in lockdown.
• This is a new issue; We've been in the house for 3 years without a problem. Nothing changed since last season, or since the beginning of this season.
• I've only seen the weird timing issue once out of tens of times the power-cycling worked.