92 chevy truck missing at constant speeds

[chevybill]

Hey guys, I am from the 67-72 part of the board. I am the one owner of a 92 c 1500 2 wd, 5spd manual with 280,000miles. I have replaced the distributor cap, rotor cap, coil, plugs, plug wires and under the rail fuel filter.
Regardless of speed, engine rpms, or gear selected when you attempt to maintain a constant speed the truck starts jerking like it is missing or fuel is cutting off. Step on the gas the least amount possible and it will level off and accelerate smooth until you try to maintain a constant speed again. I also put a can of sea foam engine cleaner through the intake via the brake booster vacum hose, following the advise of a local dependable mechanic.
I also put about 6 gallons of fresh premium fuel in with about a half can of seafoam in the gas tank.

I have not made any progress with the jerking yet and after investing 100 dollars in parts and the most of a day I need some assistance. Truck has been well maintained and most everything else to consider is original.

Distributor is original as are the fuel injectors themselves. I sprayed penetrating oil around the intake bolts to check for leaks around them and found nothing, engine speed did not change. It idles pretty slow around 600-700 rpms.

Any suggestions? Thanks for your time.