1999 Isuzu Trooper

Engine Performance problem
1999 Isuzu Trooper 6 cyl Four Wheel Drive Manual 79635 miles

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?
April 9, 2008.

Check the gas cap and make sure it on and tight
low octane fuel will not cause any of what u are having
need to have it scanned for codes and let me know what your codes are
is it due for a tune up you may also want to check that

Apr 9, 2008.