I had this problem on an 89 Chevy Impala a while back. It turned out to be worn fuel injector seals. Gas was pouring in into the upper manifold intake throttle (similar to a carburetor setup) and around the seals. The car would sometimes idle very ruff, but run smooth when moving above 5-10mph. I saw the excess amount of fuel dripping into the throttle body and thought an injector was bad. I went up to the auto parts store to get another. They didn't have any injectors in stock, but I bought a throttle body rebuild kit instead. I was bored that evening and rebuilt the throttle body anyway, figuring the injectors could be replaced later as they were on top. As it turned out the rebuild kit was all I needed.
Also, probably wouldn't hurt to replace all the plugs. When removing these, place them in the same order they were in the car. You can then compare each one and find the cylinder with the problems.
I would also check the egr valve and intake manifold air idle control. I don't remember the exact name of the air idle control thingy, but this car may have it. It is a plunger that opens and closes to adjust for more or less air into the upper intake. It takes the place of a self adjust mechanical choke.
When does it seem to run rough, hot or cold?