1990 Camry V6 Auto, Smoke, Stalling

My 1990 V6 Camry has very strange problem recently.
1) When I start the car, the idle speed is around 1600 rpm which is higher than usual. It will drop down to around 1000-750 rpm in few minutes.
2) After driving for about 10-20 minutes, sometime I will see white smoke comes out from hood. When it happens, if I stop at traffic light, I will have very hard time to speed up the car. It can only run under 30 MPH. If I step on the gas, the rmp will go up, but the car won't. There was one time that it came to a complete stop. I had my kid pushed the car, then the car moved again. It did come back to normal speed.
3) If the smoke doesn't come up, the car can drive as fast as I could.
4) The smoke comes from some where under/behind the engine block. It smell like burning ruber.
5) I changed Catalytic converter, but it doesn't help.
6) I drove it to mechanic shop, they said they could not diagnosis it without re-create the problem. They drove it out for few blocks and came back without any problem. They asked me to bring back later, since they need more time to work on it.


Any help regarding this problem is highly appreciated.

is hard to guess but please check your transmission fluids, if it is low it will give problem eventualy will burn the transmission. Check while the engine is running. if it low fill it up but don't over the hot area (overfill may damage also the transmission).

Thank you for your reply. The first thing I did when my car stalled, was checking my transmission fluids and it looked a little bit low. I did add more transmission fluids to make sure it is enough. However, it did not make any difference.