1999 Isuzu Trooper engine light is on

I bought a car, I put it low octane gas by mistake and my engine light became on everytime. Besides I noticed even before when I put low octane gas that
sometimes while driving suddenly car loses power and starts little slow down if I accelerate more it gains power but feel like something push it back and then it becames stable again and when I put on neatral while driving my rpm becomes so low , or even jumping low -high almost shuting off engine and then became stable, but when a car parked idling is stable. When I put after high octane gas, infew miles my engine light went off and stayed that way a while. I drove about 20 miles for commute and suddenly while driving engine light became on againg and now it still on. What is a problem in car and what shall I do to fix it and how mush it will cost?