Our 02 Yukon XL with the 5.3 flex fuel model runs rough at an idle sometimes, then others it idles high and going down the road sometimes it feels like its skipping. The problem is that it doesnt do it all the time, it comes and goes which is making it hard to find the problem. We have 150,000 miles and have done the following: replaced fuel filter replaced intake gaskets (there appeared to be a leak) added fuel injector cleaner (twice) My friend who is a mechanic, has a diagnostic tool and he keeps getting alot of misfires from various cylinders, but more misfires seems to be coming from cylinder #2. He also gets a O2 sensor code. Any ideas? Thanks to all in advance! John - - - Updated - - - I should also note that we put a fuel pressure gauge on and it reads steady at 45-50. - - - Updated - - - spark plugs have about 55,000 on them, wires have only about 20,000 miles on them. We did exhaust manifold last year because some of the bolts were broken.