1997 Isuzu Hombre P0300 Random Misfire code

For a long time, I have been getting 0300 codes . The truck has about 130K on it. I have changed the wires, pluds, two O2 sensors, distributor cap, rotor, fuel injection system to the upgraded spider. The truck drives fine, them stumbles for a few miles or so. After a while, the SES light comes on and the engine drives fine. After a while the SES turns off. then again in a day or two it comes back on. I have taken it to a mechanic with 20+ years of chevy experience and he has done most of the work. He can't figure it out anymore and dosen't want to see the vehicle again.

I would rather not sell the truck but since this has been going on for about a year, I have reached a level of frustration......

My crystal ball tells me tis is the 6cyl 4.3L and why a chevy tech is working it.

Also let us know-
ever any other code?
parts used are AC delco?
mmodifications - ? especially remote start or aftermarket alarm

there are some ideas we can float knowing the above will help us