I just got my 99 rodeo v6 back from the shop(second time it has been there in the past week). I took it in last Wednesday because of a CEL and a rough idle(felt like it was going to stall). I got it back and the next day the same thing, rough idle and CEL. When I had it there the first time the CEL was PO171 and they replaced the O2 sensor. Also they cleaned the ERG, changed the air filter, fuel filter and the spark plugs(NGK). It was a full blown tune-up. I got it back and as I stated it needed to go back. Today I took it back and they said a plug was bad causing a misfire(CEL on and blinking). I picked it up earlier and it was fine again. Just used my truck and the CEL is back on and the rough idle is back. It is very uncomfortable to drive. I spent $600 (money is real tight right now) last week and today it was free obviously, but it is still not running correctly. I was wondering if anybody has any ideas on what I need to look at. I am going back (I trust these mechanics I've been using them for years and I used to work for them as a parts deliverer) but I am at a loss I don't have a clue what is wrong or where to look. Anyone point me in the right direction as to what might be wrong. Could this be caused by a bad intake manifold gasket?

Thanks in advance
Marc