Need help with reduced engine power on 2009 impala

I have a 2009 Impala, and keep getting reduced engine power, traction control, check engine etc, when it is raining or slushy. we have had throttle body, throttle position sensor, and wiring harness has been changed, and it still does it frequently. Dealership resets code and can't make it happen when we take it in. I've spent thousands now at both GM dealerships and non GM service centers. Please help