doublex

Hello My engine vibrates really bad only when it gets warm from 50C to 90C shakes can be felt inside the car, rmp hoping up and down, mpg decreases significantly, fuel smell and white smoke from exhaust (only when it's shaking). When it's cold or warmed up to 90C it's alright. Rmp steady, starts easy and everything works as it should. No faults on dash and computer didn't read any faults either. Why it's only shakes from 50C to 90C? Can someone help diagnose the problem I'm having?