My approach (not an electrician, just a guy with a meter) would be to start by mapping the circuit (find all of the outlets and fixtures that are on it), then unplugging EVERYTHING that's plugged into that circuit and see if that has an effect.
Next up, I'd kill the breaker and open up the switches and take a look. See if there's evidence of burning or arcing, or if one switch feels different than the other. I'd probably disconnect both switches (safely capping all wires), then turn the breaker back on and verify (using a meter) that the voltage is reasonable at all the outlets and other fixtures, and see what happens in the bedroom.
Basically if that tells you nothing, then I'd either start pulling outlets and fixtures off the circuit (kill breaker, remove outlet, capping wires, breaker back on) until I figured out what the heck was wrong, OR I'd just call an electrician.
If you are the landlord I would have another electrician come out to check it. It be something like a factory defect in the subpanel or main panel. Every lug could be tightened and torqued to factory specs but the lug might be stripped out.
If you are the renter then I would make it clear to your landlord that this is a serious problem and you want if fixed now.
Which ever you are I would have every lug on the ground, neutral and hot sides torqued down to factory specs. Get the electrician to do a thermal reading while the panel is hot and having the problem. This would surely cause extra heat where ever it is happening. If the electrician does that and nothing changes have PGE torque everything on their end and do the thermal reading.
I had the same problem and when they removed the meter and left I removed the Edison side covers and one of the lugs was never tightened on their end but it was a factory connection that was the problem.
Edit On questions on what to use to torque a circuit breaker, see this. Each breaker has a label on it with the torque rating on it. At one time there was talk of inspectors carrying torque wrenches with them to spot check circuit breakers, but I know of no electricians own a torque wrench. When I ask they say just tighten them down as hard as you can and they will pass any test. My guess is that most residential breakers use about 20 to 25 foot inch lbs per square inch.
Most of the time torquing happens is in industrial plants where they want to get the maximum life out of everything, including breakers. This is also why thermal cameras are sold to electricians. Heat spots are bad and most of the time it is a loose connection.
In the OP's case I would definitely use the phrase Torque to specifications, especially if he puts any thing in writing. His problem can wreak havoc on electronics and motors.
Best Answer
Call your power company and report an outage
When half your 120V circuits are dead and half work -- and the dead half comes back on when you turn on an oven or water heater or dryer (but those things don't work) -- that means one of your hot wires has failed coming from the power company. It also may be a problem with the main buses or main breaker.
Now, if your 240V loads like water heater work fine, but lights alternate between too-dim and too-bright/burning out, that's a more serious problem - yes, more serious than an outage. It is a lost neutral, and it will cause your 120V loads to wobble above and below 120V, which can fry appliances and potentially cause a fire.