I have a 1996 GMC Jimmy with around 126K miles. I bought it used about 2 months ago, and began noticing problems a few weeks after purchase. After putting a bottle of fuel system cleaner in, I noticed difficulty in starting the motor. The motor has always started and never left me stranded, but it began starting on the second or thir attempts. One day while driving home from work, at around 65 mph, the motor began "chugging" and almost bucking while I attempted to accelerate and the check engine light came on, and stayed on til the next morning, and then it shut itself off(the light). I have not changed spark plugs, fuel filters, or anything outside an oil change. What could be my problem?