loss of power

I have a 1988 g20 3\4 ton van with the original motor and trany when I bought it it's response time on the throttle was great i changed almost every sensor in it and changed cap&rotor spark plugs and wires and frequent oil and trany fluid changes it don't tap or knock or smoke but I have loss of power and when i drive it for 15 to thirty miles the check engine light comes on but shuts off when i turn it off and on again until I drive that distance again. I'm lost no one around hear has the A-2 diagnostic module and i don't know where else to turn or what to fix so please you are my last hope what would be my delay.