Trouble with my 1989 Chevy G30 with a 5.7?

[bruce]

I have a 1989 motor home with a 5.7. I have been having trouble since last year. I was on vacation and got gas and jumped on the turnpike. About 20 miles down te road the engine started missing and the check engine light came on. I made it to a chevy garage and they told me the codes were for lean fuel and ignition problems. They changed the fuel pump and we test drove it about 10 miles and everything seemed fine. We set out again and about 20 miles away and it started again. It will miss and buck and even shut off if you let out of the throttle. I made ti to where I ws headed and talked to a GM mech. he got me the information on the codes and we changed the entire ignition system. Rotor, dist, coil, ecu and drove it about 35 miles without trouble. Then next day we headed for home and got about 40 miles into the trip and it started again. I made it to a Auto Zone and changed the knock sensor, got about another 15 miles and it started again. We stayed on secondary roads and made it home. since I have changed plugs, wires, had it tuned up, changed the fuel pump and filter. I drove it again on Sunday and made it 40 miles stopped and got gas and started back got about 20 miles toward home and it started again. It is running he best it has in years right up until it starts to miss and buck. I am at my wits end, anyone that thinks they might have a clue about where to look next i would appreciate it.