computer light

I have a 1993 Buick 174,000 + miles on it V6 3300 engine. When drive on interstate or other 4 lane roads at 70 miles an hour going up hill check engine light will come on and the engine will surge when light goes off whether by pushing in cruse control button for a couple seconds or by going down hill. I have changed plugs , irs cables, o2 sensor, new water pump. and ne buffer around time chain. Any one have any Idea what is causing this and how to fix been to three different mechanics with little success. Thank you.