5.7L rough Idle "cough"

I have a '94, 1500, with a 350 it has 115,000 miles and when it idles it has a cough, like a cylinder is not firing, but it doesn't do it all the time.(most of the time but not always) it runs perfect while driving but at idle it's rough. The Idle is very low also (lower than I feel it should be) it has never died on me though. I have done all the basic stuff for a tune up and it didn't help at all. I have not changed any sensors or anything like that. So if anyone has some advice for me to try or has had similar problems it would be much appreciated THANKS!!!!