'98 K2500 acting goofy

[jfnar]

Here’s the situation. I bought this ’98 K2500 about a year ago. It has a 350 engine and about 150K miles. It seems to be a pretty good truck. One of the things that impressed me when I test drove it was that the transmission was so smooth that you could hardly tell when it shifted if you weren’t looking at the tach. About two months ago, I drove it a couple of miles to my shop and it would jerk pretty good when it shifted. The next day it shifted smoothly again. Okay, jump ahead to last Saturday; my dad and I are heading to the deer woods with a trailer with two 4-wheelers on it and we get about 10 miles down the road and I see the “service engine soon” light come on. The very instant that the light comes on the speed starts dropping off. It was on cruise so I pressed the pedal down and nothing happened. It continued to slow down so I turned around and headed back home. It would only do about 25 mph and pressing the gas pedal down didn’t help, it just seemed to run a little rougher. It was running about 3000 rpm. I turned on the 4-way flashers for safety and I noticed that every time the flashers came on it would lose a little more power and then come back a little when they went off. You could feel it pulsating as the flashers cycled on and off. I turned the flashers off and it smoothed out but still only 25 mph. I tried turning the heater fan on and off and it too would affect the engine speed. Just before we got home it suddenly started running right again. The next couple of days it ran like a new truck. The “service engine” light went off the following day. I took it to town to O’Reilly’s to see if they could hook up to see why the “service engine” light came on and they told me that since it had gone off already their reader wouldn’t tell anything. So I start back home, everything working great, then I noticed that it started shifting hard again. I got onto the highway and after a couple of miles it shifted out of OD into 3rd gear. Engine still running fine doing 55 but it won’t go back into OD. I haven’t driven it since then. I like to work on my older trucks when I can but I’m not up on the later computerized stuff. My gut tells me that there’s something going on in the electronics. I’m swamped at work right now so I’ll have to take it to a garage to have it worked on but I’d like to have some idea what’s wrong before I take it in. It gets pretty expensive to pay someone to swap parts until they accidentally fix it. Thanks for taking the time to read all this. Any suggestions to help point me in the right direction will be much appreciated.

Jeff