What could be causing this electrical issue?
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.