The easiest way I know how to figure out whats burning is look. Go out and do what ever you need to do to start the smoke again in the cab. (you talk like this is a regular occurance) when you see the smoke get out and crawl underneath and see where it's coming from. I have a feeling there is something on your floor melting. I would think that a tranny that has gotten hot enough to smoke, repeatidly, would show other issues as well.