1997 Other Chevrolet Models Check engine light frequent

Why, after replacing plugs rotor cap wires and oxegen sensor, and engine light reset, my truck, at any particular time, will kickup about 250 to 300 rpms while driving at highway speeds. This truck has always done this and no one can find the problem. The best it ever ran was 2 weeks ago after a service tech reset the timing which he claimed was off over 10 degrees? I know that don't seem possible but it is what it is. Anyway the truck was running great after he worked on it, but after 575 miles, it went right back to it's old ways.I was running 2000 rpm's cruising at 70- 72 then rpms kicked up again to 2,300 and roadspeed dropped a bit and gas milage plunged?Since then, sometimes engine light comes on. Sometimes not. Am I driving a common known chevy lemon?Distributor problems have plaqued this vehicle. Crankcase moister may be building up in cap.I replace it often. Today it might run well for a little while then change after driving 20, 30, 50, miles, who knows?This truck has spent 0ver 75% of it's life with engine light on. Always a problem to get it to pass NYS inspection rigors. Frustrated and very dissappointed in GM. Codes say missfire.