I've got a 98 Tahoe with the vortec 350. When it reaches around 60-70 mph, and shifts into overdrive it will run rough, and the engine shakes, if I drop it into third it will smooth out and be fine, plus if you mess with it it will run smooth after a while. If you let it run rough long enough, it will throw a code for number six cylinder misfire. I have replaced all of the fuel injectors, the fuel meter and the spark plugs, all of the plugs looked alright, but it had ran for a little while without missing. The only other thing I have done is switched the ignition module with the one from my 98 pickup, same problem in the tahoe and the pickup runs fine. Anyone have any other ideas what may be causing this?