Loss of power

I have very limited mechanical expertise so my descriptions may be challenging.

The car starts great, can go forward and backward, I can drive it and feel when the transmission changes gears (same as my second car). After it runs for a while (I run it maybe 10 miles to work and 10 back) there is some smoke (clearly not steam) coming from the right (drivers side) not barreling or rolling but enough to be viable when I arrive to or from work. While driving there are times (getting more frequent) I lose the ability to move forward (almost as thought some one was in front of me and pushing/holding me back.) When I stop and go at traffic lights. My first assumption was transmission so I called a friend that works for a well known transmission repair facility. He came by and looked at it and said its not the transmission but passably t he catalytic converter. I took it to another well know exhaust repair shop and they could not find any problem in the catalytic converter or any other part of the exhaust system. So I decided to take it to another/different transmission/every thing shop. Left it for a day of diagnostics. They came back with a “there’s nothing wrong with the transmission. That surprised me. A transmission shop that would tell you your transmission is good and not try to get me to spend a lot of money. They said it was the catalytic converted. So I again took it to another shop (national) and they can find no issues with the catalytic converter or exhaust system. I am lost on this one. It is intermittent but happening more frequently and no one can tell me what the problem is. Again the car is twenty years old so I don’t want to tie up thousands of dollars into it but I also don’t really want a new car payment.

Any Ideas?