1999 Toyota Corolla Engine

Automatic transmission
716,840 miles
HELP ME! I know some stuff about cars, which is still next to nothing and I don't know what's wrong with my car. It started right after I got the oil changed about 2 months ago, and has been slowly getting worse. At first, it would just have a hard time accelerating up a steep hill, but after pressing harder on the gas, it would be fine. This has devolved into a serious problem. Any time I slow or stop, it almost stalls, and (to get out of the way of the angry folks behind me) when I step harder on the gas, it will move, but sometimes starts making this klunking noise. Other times, it drives just fine. It really doesn't like hills, which is a problem considering I live in Duluth (all hills).I've had the oil changed since, no difference. And 3 or 4 times, I've added bottles of that gas line cleaner stuff that is supposed to clean gunk out- no change. It seems like a problem with the car not being able to get enough gas. Any ideas or suggestions would be life-saving! Thanks!