1994 gmc 2500 stalls

My truck begins to stall after approximately 20 miles, just
stalls while driving. As I roll, I can put it neutral turn the key
off and back on starting it right back up again and carry on
by putting it back into drive. It will do this over and over
again.

But it doesn't do this till after 20 miles.

I have changed the fuel filter, fuel pump, and something in
the distributor cap can't recall the part.

The mechanics say they can't run diagnostic because of
lack of computer. It's a 1994 gmc Sierra 2500.

Any suggestions would be great!