2002 Chevy Malibu acceleration and overheating problem.

1. 2002 chevy malibu
2. A little over 112,000 miles
3. Automatic transmission
4. Front wheel drive
5. Not really any strange sounds
6. When overheated fluid poors out like a leaky faucet! Other than that no noticeable fluid.
7. In order- have tried fuel injector cleaner additive, changed transmission filter, had radiator and lines flushed and refilled, checked all fluid, and changed fuel filter

We have been having an acceleration problem for about 2 weeks now. It's like it is having trouble shifting on its own. Really hard to get up to 30 mph but once it gets past there fine(In the overdrive gear too) until you stop and have to accelerate again. It overheated yesterday to the point where fluid poored out and steamed something awful.
We took it to a mechanic here in town and he said that there was a lot of air in the radiator so he checked all the lines but they were fine and just flushed it and replaced the fluid. However, my mother failed to inform him we were having the acceleration problem! Grr But it is still running pretty hot too. Driving home today it still had trouble getting up to 30 mph until I started shifting it myself and the heating gauge was over half the entire drive. Could someone give me a better idea of where to look for a problem? I don't see how these two problems could be related.
I will post any other information you need just let me know
Thank You