1993 Isuzu Rodeo Transmission

We have a 1993 Isuzu Rodeo 130,000 miles, V6 and the Transmission light keeps coming on. We had a transmission flush done about 2 years ago when the light came on and it stopped. We just had a tune up done in August and had our mechanic run the transmission on the machine and he says there is nothing wrong with it. They reset the sensor on the oxygen sensor, but it was not changed. Sometimes smoke comes out from the exhaust and it smells like something is buring or leaking, but the mechanic has found nothing wrong. I just don't want to turn it in to a transmission place and told we have to do something that could be costly.