Sluggish Acceleration; rough idle

1998 Chevy Cavalier, 117,000 miles, 2.2, My problem is this: When cold, the car works pretty good. It used to take awhile for engine temperature to reach normal heat (mid-range), but now it only takes about 5 minutes of running for it to reach the half-way point, and it doesn't get any higher. The hotter the car gets, the more sluggish it gets. When at a complete stop, if I push the pedal all the way down, it backfires. Once it reaches about 25-30, it runs okay, but you can still tell that it isn't running smoothly. When at idle, it is a rough idle. A friend's husband has been working on this car for about 6 years and he's checked everything. There is no error lights that are coming on. He's changed the spark plugs, wires, checked the air filters, fuel filters, compression is good, there are no clogs anywhere that he can find. He's suspecting the timing chain and will check it out this weekend, do you have any suggestions what the problem could be? Thanks for your website. It's great! :D