1992 Chevy Corsica This car has seen better days.

Engine Performance problem
1992 Chevy Corsica 6 cyl Front Wheel Drive Automatic 178k miles

I have a 1992 Chevy Corsica. It has a cracked block, but the block was welded. I bought it from a friend in June of 09. I didn't spend much initially, but since I bought it I've spent about $300 semi reviving it. Which is much more than I spent on the car. Now its idling is fairly sporadic when I first start it up, but it doesn't die. The engine runs rough and feels as though there is a loss of power, and the idle jumps up and down. However, if I shut it off before the first 1/2 a mile or so, it has trouble starting again. It will turn over, fire, but lose its idle. Its idle isn't really bad. I guess its bad enough for someone who pays attention to notice. But it affects the braking sometimes. It feels like the car is still trying to accelerate when the idle is up, but goes away once the idle goes back down. It also sputters before going from a dead stop (i.E. Stopping for a stop sign) when I use the accelerator. After the car has been driven for about a 1/2 of a mile, it accelerates like normal, and doesn't stall before accelerating. But the idle is still irregular. The car also runs at a strange temp. The gage is accurate, but the needle tends to hover around the 3/4 line. I replaced the thermostat a month or so ago because the one in the car was not opening all the way, causing the car to overheat. I replaced it with the spec t-stat for my car (195degree setting) It is in there the right(i didn't know it was common to put them in backwards). For a while after replacing, the gage was really sporadic, fluctuating between 1/4 and 3/4 marks. Now it has plateaued around 3/4. (Is that normal?) But it has not overheated on me since. With the recent rough idling/accelerating, I tried a fuel injector cleaner. Didn't help. I replaced the air filter and fuel filter. It helped a bit, but its still not running right. My next step is plugs and wires, but i'm wondering. Would the fact that the engine had overheated, blow the welded crack on the block? It would explain the loss of compression and irregular running habits. I was thinking maybe the camshaft sensor, but I haven't seen the 'service engine soon' light once in the 4 months I've had the car. And I'm pretty sure if it was a sensor, the light would register a problem. I know enough about cars to know that there's something wrong, I just can't figure out what. I'm sure there are many problems. But the most desperate right now is my engine stalling out and idling sporadically. I'm out of ideas, please help?