Rough Idle Issue

[suprajim]

Hey guys, I need some help with a rough idle issue. I've searched, but couldn't really find any other posts with the same symptoms. My car (92 Turbo 7MGTE A/T) starts up fine and idles at about 1200 RPM for about 30 seconds then it drops to about 700 and sounds like it wants to die, but never actually does. It drops RPM's, but then recovers and goes back up to about 1200 before dropping again. The only code I have is 21 for the O2 sensor. I had recently replaced the vacuum hoses with a new silicone kit, so ruled a vaccum leak out. I thought maybe I had a misfire from oil on top of the plugs, but had replaced the #3 head gasket and replaced the plugs (NGK platinum gapped to spec). The wires are NGK and I tested them with an ohmmeter when off the car and resistance was well within specs. I also checked the ignition coils and they seem fine as well. I drove the car around the block afterward to burn off the oil that dropped in the cylinders after pulling the spark plugs and seemed to have plenty of power and it doesn't sound like it is misfiring when I get on the gas, but the idle issue remained. I adjusted the timing, which was at about 25*, back down to the factory spec of 10* and noticed no change at all. I don't see any smoke and the previous owner replaced the head gasket with a metal one about 10k miles ago. I'm going to check out the ISC valve next, but didn't know if anyone had any other suggestions.