2000 Oldsmobile Intrigue noise when changing gear

Yesterday, I drove down a highway headed to my hometown (about 20 miles). When I got intown, I was at a stop light, and saw my RPM's go down less than 1. As I drove (slowly), I heard a bang as it was changing from 1st to 2nd gear. Mind you this is a automatic. When it changed to 3rd gear, the car stalled. I was right by my parents house, and parked the car for like 2 hours. My Stepfather was an ex-mechanic, and took the car for a test drive when he got home. He drove around the neighborhood with no problems. So I then drove home, hoping the car wouldn't brake down, It didn't. I had no problems on the way home, and when I got home, I parked. About an hour later, I started driving the car, and noticed a mild thump when the gears changed. So luckily, I was right by the dealer. So I dropped the car off and let it sit overnight til the dealer got there. He drove it around a few blocks with no problems. He said it wasn't the transmission, b/c there was no fluid leaking, and I would have noticed this before. He suggested that it was water in my gas tank, and to buy dry gas. I did this, and on the way home, I started hearing the thump again. What could this possibly be? If I take this to another mechanic, what should I ask them to do? You guys were so helpful with my last problem, thanks!