What's wrong with my car? is it my ECU?

I have a 2004 Chevy Cavalier (base) with 211,000 miles.

Last week it started acting strangely, it was very rough at idle, shaking, and stalling out and I had a hard time getting it up to and staying steady at 55 mph. So I immediately took it in.

After 4 days at the mechanics shop I was told they had no idea what was wrong with it and that one of their guys had spent 5 hours straight looking at it. A day later I was told by them that my timing chain was junk.

Not even five minutes after I got the chain replaced and drove off the lot the problem came back.
I went back to them and told them what happened, but that's a whole other story. So I drove the car home (problem still not fixed) until I could figure out what to do.

Bare with me it's been a long week and I don't speak mechanic very well :-)
This is what ended up happening to the car:

We had an all point inspection done there are no clogs or leaks in any of the lines. The plugs are new and working well and so is the air filter. The fuel filter is fine and there is nothing wrong with the fuel pump or pressure. The fuel injectors are nice and clean and the battery, coil pack and alternator are all getting proper voltage. The TPS and MAP sensors were tested and those were both bad after new ones were put in the car ran fine and the problems vanished but when the computer had to be reset for the check engine light the problem came back.

This time was different the car ran super smooth, got up to 60 easy breezy, but still had a rough idle and slight stalling. While idling the RPM's would go from 1000 to 500 in about two minutes.

Soooo now our third mechanic told us it was the o2 sensor (exhaust side) and everything else was great despite my high miles so I replaced it, and once again it ran great until the battery had to be pulled out (unrelated matter) And now the problem is back - 4 cylinder car runs smooth but idles low and rough, slight stall.

My fourth and final mechanic looked around and listened to what i had to say. He told me that the engine is randomly misfiring and everything looks alright regarding that and it's no particular cylinder or plug or anything... He also told me he thought it was a wiring problem or the ECU.

I just don't know what to do... Should I trust this new diagnosis (he didn't hook it to a computer or anything.) I live in a very small town and I don't have the means to keep going to these mechanics that only have cash on their minds or very far away. So any info or experience would be great. It's a nice car and my families only car and I would like to keep it a bit longer.
My husband took the battery out (after it tested fine) because he misunderstood me over the phone, he thought that I was buying a new one at the auto parts store. It was a silly mistake so he put the battery back in after about an hour and re tested it. all is good with the battery. But it does seem like things go funky after the computer resets itself...