Stumbling after start up

[Wgesnerjr]

1985 C10, 2WD, LWB

2003 Tahoe, 5.3, DBW/non-DOD/Flex-fuel/DBW. Return fuel system, 4L60E, New 1987 baffled tank, New sending unit and filters, New Fuel Pump in-tank AC Delco EP381 61.6 PSI., Stock Plugs-new wires. Flex removed during reprogramming.

Symptom/Condition: Upon initial driving, engine runs rough. Bucking, stumbling, etc but not trying to stall. Just severe loss of power. No Service Engine Soon light. No codes. No backfire. At idle, you can’t tell it’s happening until first acceleration. Give it lots of throttle or a restart and it goes away. Voltage seems normal. Gas at a top tier station, gotten several tanks there before. (In multiple cars) had ¾ tank of gas during issue.

---So after 2000 trouble free miles on my swap, I have developed an issue that comes and goes.

Drove the truck Saturday afternoon, no issues whatsoever. The issue started Saturday Night. Truck parked flat in bottom of driveway. Started up quickly and on the first try as always. Sounded normal. Tried to drive up to the top of the driveway and it was bucking and stumbling. When I got to the top, I turned the truck off and waited 30 seconds. Upon restarting, the truck found an idle and drove with plenty of power, like normal for the rest of the trip. Went and came home without shutting it off.

Sunday came and the first drive to my buddy’s house, everything was normal. Parked the truck in his driveway, nose up. Starting up to leave was an hour later and the problem started again. Drove down his street, pulled over and did the whole shut off/restart and power was back. All was good again.

Next stop was about 20 mins. At O’Reilly’s. Upon restarting, I did not notice the problem immediately as the truck was parked nose down and I put it into gear and coasted to the exit. The store is on a 4 lane highway so I pulled out when clear and the problem was back. Bucking, stumbling; almost like it was missing but like 4 out of 8 cylinders were not firing. Planned on pulling over for the shut off/restart when traffic started bearing down on me. As not to get run over (and because I was getting annoyed) I dropped the pedal and the truck took off and got up to speed like normal. It found it’s happy self and responded the way an LS should. Turned off the highway and it was purring like a kitten for the rest of the ride.

Ran two more errands and parked in a flat parking lot both times. The problem came up on one of the times. A quick step on the throttle and it issue was gone for the rest of the ride.

No codes stored in computer. Doesn’t happen every time, but is getting more frequent. Thought it might be choke issue.

Any ideas? I am worried as this is our second car and do not need my newly licensed daughter having a problem out on the road.

I can do small diagnostics at night, but by the time I get home from work with the time change, it will be dark. So bear with me on replies. More in depth diagnosis would wait until the weekend.

Thanks