hrmmms i was abt 2 say...any chances the power wiring is getting soaked by those "puddles" or "heavy rain" because if so then the electric current will tend to follow the water n that will take the power from ur car...if ur engine is recieving the right amount of power it could lead a vibration or jerking???

hrmmms one of my suggestion is lift the car up, spray it wif heaps of water n chuck it on the dyno n see if there's any vibration or different reading in the car power