Greetings. I have a 95 chevy 1500 4x4 that I purchased new. It's a member of my family and ahas 367000 miles on the odometer. About 30000 miles ago I swapped the original 5.7 for a 96 model vortec 5.7 from a tahoe. I purchased a gm performance parts intake to allow the use of the 95 computer and tbi. The swap went well and I was rewarded with increased performance... for about 28000 miles. While returning home from work one night at 2 am, the truck developed a pretty serious miss/stumble even at 70 mph. I pulled into a station and checked everything I could think of nothing obvious was visible I found that with some significant effort I could rotate the distributer and advance the timing. It smoothed out a little and I was able to get it home. When I had time and daylight and I dug deeper. No trouble codes were set (OBD1) I set the timing according to my haynes manual, with a timing light and the spark control wire under the passenger side dash disconnected. The manual specified 0 degrees advance . I plugged the wire under the dash back in and fired it up. No change. I read the plugs, they indicated a rich condition. I changed plugs and wires, cap and coil (Not all at once) still no change. I changed the coolant temp sensor and rhe o2 sensor, no change. I discovered that unplugging the wire under the dash smoothed it right out. I drove it like that for a couple of weeks, but fuel consumption was way up (8-10 mpg) I parked it for while and thought about it. I changed the spark control module in the distributer (according to the manual most of these trucks have an external scm, not this one) no change. I thoght about it some more. I changed the knock sensor-twice-you guessed it no change. I don't know where to go next-Ive checked for pinched wires to 02 sensor and knock sensor and probably a bunch of other stuff I've failed to mention. Any Ideas? Thanks in advance for your time.