2008 mazda cx7 grand touring Power loss (deacelaration)


MAZDA CX7 Grand Touring
I am having 2008 mazda cx7 grand touring.While driving at around 30~40 MPH my car is loosing power and decelerate. There was engine light.I went to the dealer and they replaced O2 sensor.Engine light is gone but that didnt fix the decelerate problem.. I went back to the dealer and then they replaced catalytic Converter but still didn't solve the issue.. They have no clue now.. I already spent $2K to get it fixed but this problem is not resolving.Does anyone has the similar issue and know a FIX?