running funny after head gasket job

i finally got the 93 quad 2.3 DOHC back together after the haed gasket job. the good news is there is not any more water getting into my oil...
When i started it for the first time it ran fine until it warmed up and then the idle started fluctuating from normal idle to lower idle and back(pretty quickly back and forth). i open the coolant resivior and it has pressure, even after the car cools off. i tried running the car with the coolant cap off and it seems like exhaust is seeping in there somehow. any ideas of what i should be looking at to pinpoint the problem?

Did you check the head for cracks? When I changed mine the head was cracked in 3 places that we could see, no telling if there were any more or not, I had the head that I came across pressure tested before I started the job, so I knew it was a good one.

That's what it sounds like. It sounds like a cracked head, as a brand new gasket shouldn't be causing this - unless of course they failed to follow the correct torque sequence / specs.