2006 Chevy Uplander Reduced Engine Power

We continue to have issues with reduced engine power with our Uplander. It seemed that after it rained we would get reduced engine power. The light would come on, sometimes before I backed out of the drive way, sometimes after I drove to for a minute. It would stall and I'd lose all power. I'd have to restart, wait a minute and hope I could move a few more feet without repeating the process. More recently it was happening when the weather is fine.

We took it to the shop and told that the computer read that it was a throttle problem. They fixed something with that and something with the gas pedal. We got the car back last night.

Well, it rained last night and I didn't even make it out of the driveway when "reduced engine power" happened again.

Help! (Please).