1993 Other Jeep Models 1993 Jeep Grand Cherokee Intermittent

I have a 1993 Jeep Grand Cherokee that will intermittently lose power and shake. I will be driving down the freeway doing 65mph and for no apparent reason, the engine will shake and sometimes the check engine light comes on. The shaking gets worse if I step on the accelerator. Then for no apparent reason the shaking will go away to sometimes for a few hours and repeat again. I notice on the mileage computer that the miles/gal goes down when the shaking occurs. The car even with the shaking will maintain highway speeds though. Please advise on possible causes.

Thanks,

David