I have a 1999 GMC Jimmy SLT 4WD with 87,XXX miles. The other day I ran to the store and back (about a 7 mile round trip). Previous to this trip, I havent had the need to go anywhere for 2 weeks because I wasnt in school. I let the car warm up for a couple of minutes, and then left the house. I have the climate control and I had the temperature all the way to 82 and the fan on high. Usually, it heats up fast and I have no problems. Well it wasnt warmed up by the time I got to the store, and by the time I left, it still wasnt warm until I was accelerating. I immediately looked at my temperature and oil pressure guage. My temperature was running about 225-240. I then looked at my oil pressure guage and it would fluctuate. When I was on the gas pedal, it would run very high, then immediately drop when I let off. We got home and me and my brother immediately checked the compression. The cylinders were reading as this, four of them were 150, and the last 2 were at 115-125. As we checked the cylinders, we saw that the spark plugs were stock platinums, so we changed those. Then, we discovered the main problem was that intake manifold gasket was leaking. So we replaced that, let the RTV settle to for over 24 hours, and put everything back together. We flushed the fluids, and put new fluids in it. The radiator holds fluid, and there arent any leaks. We started it up, and at first it seems that its burning a little bit of oil and the oil pressure was at about 50, where its normally in the 40s. We let the engine run for a good 15 minutes, and it started slowly heating up. But, its idling pretty rough and we cant seem to figure out what the problem is. I did a search, and Im not sure but could it be either the EGR valve, EGR gasket, or some other major malfunction?