Problems with the power lock switches, lock mechanisms and actuators seem to be more common on 2G cars than 1G cars. Some of the problems have been caused by loose window switches, while others have been blamed on contamination of the lock mechanisms with debris, or simply worn out actuators.
Also, one DSMer reported a factory build problem on certain limited '99 model cars. Apparantly, the factory applied too much sealant to the inside of the door. The extra sealant (or 'gunk') can run and leak into the door locks, causing them to progressively freeze up or otherwise misbehave.
Unfortunately, the only solutions are to repair or replace the affected parts. Loose window switches can usually be fixed by repairing the screw mounting points that hold them in the door. Actuators generally must be replaced, as do door lock mechanisms.