2001 Toyota Camry Car exhaust lets out a cloud of smoke whe

About 1 mth ago, I took my car to the shop to repair a serious oil leak. A new gasket, water pump, timing belt later, I get my car back. A few days later I note that after getting off the highway and onto regular traffic speeds, the car would hesitate a bit (like a slight tug) upon acceleration from the first full stop, then a huge cloud of smoke would come out of the exhaust. This would never happen while strictly in regular street traffic.
I took it back and supposedly, they check the car and said that all it needed was some transmission fluid, because somehow I had none. I get my car back but the problem with the hesitant engine and smoke billowing after highway driving persists. I call the mechanic and he is telling me that I probably need a tune up.
I am very upset to have to take the car back what I consider an incompetent and perhaps crooked mechanic. What is the problem here.