I need Tech Help. Can someone help me please.

I have a '97 E230. Recently the front power window, power seat adjust and wing mirror adjust (both sides but not necessarily together) would go dead. I've made more than 10 trips to the workshop and each time they opened up the door panel, disconnect and re-connect the connector to the controller unit, everything would work fine for a couple of days. Then the entire episode is repeated.

After a few trips, we discovered that by just disconnecting and reconnecting the power to the controller unit inside the door panel while the ignition is on would "reset" things, so apparently something must be causing the controller to lock up. We could achieve the "reset" by pulling out and re-inserting the fuse while the ignition is on. They've changed new controller units, the wing mirror motor, and the power window motor (because these are the only units used before it went dead, didn't touch the seat adjust) but the problem persists. What else do I need to change? Why this happens only recently to both front sides, but not necessarily together, is very puzzling too.

Exasperated and need expert help!