Check engine light-1998 Mazda Protege LX

My car showed check engine light two months back. I was somehow delaying the trip to the mechanic for some time after that. Meanwhile I found out that check engine light disappered automatically then it reappeared again after driving for 50 miles and this cycle continued for some time. Disappearing and reappearing of check engine light quite confused me so finally I took it to the mechanic for the diagnostics however he found "several" problems in my car, so instead of repairing/fixing those problems he thought may be he should reinsert the chip and when he did that the check engine light disappered automatically. Then he asked me to drive a car for a while and take to him again if light again shows up. The check engine light indeed showed up but I did not take the car to the mechanic. I am confused about this. Can you explain whats going on with my car?

Thanks