Transmission problems, gas cap, and rotten egg smells?

rotten egg smell, sounds like a cat' converter. if it was/is plugged it could cause some back pressure and throw off your O2 sensor, you should have this checked. as regards to the gas cap, yes if it is on wrong or the seal is worn it will trigger a "check engine" light, it may need replacement. as for the tranny, what is the milage on the car and each rebuild? If my tranny went out 4 times I would get the whole car checked for something else being wrong, (something bent, bad rebuild --> this will only count for one or two rebuilds, after that there is something else wrong) How is the car driven? (city - stop and go or racing) do you come to a complete stop before changing direction? (this is a very common issue for blown trannys). But if it is the tranny have a new mech to have a look at the whole car to see if something bigger is causing these issues. it never hurts to get a second opinion. I hope this helps, best of luck.