02 gmc service light comes on when pulling a load

I have a GMC 6cyl m/t . My problem is when I pull my boat ( a 17 foot tracker) my service engine light comes on when going up hill. It starts running rough and losing power. then when I make it over the top it starts running smooth and the light goes off. Lately it comes on when running the a/c without pulling a load. Changed plugs and dist cap and rotor button recently. Thanks for your help.

Just saw your post and you may have it resolved by now. I had the same problem with my 96. Replaced the 02 sensors and that solved my problem. Good luck.