1997 Mercury Cougar Recently repaired car, shakes.

This vehicle is in fairly good condition. 2 weeks ago, I acquired it after having the fan motor changed, the back right tire replaced, and the oil changed. It was running great. I drove it from where I bought it, Tennessee, to New Orleans where I live. About 370 miles. It ran fine, no troubles.

About 4 days after the trip it began vilently shaking. I had put regular unleaded in the tank, let it get almost down to empty before I refilled it with reg unleaded. Thats when the trouble started. I contacted the previous owner and they informed me that it shakes sometimes when not given mid grade gasoline. Since then, I have put mid grade in it, which has done nothing to improve the shaking.

All of the guages read fine. The knocking/shaking is constant. It gets worse when the A/C is turned on. Previously, the A/C blew extremely cold, now it blows warm sometimes. Basically the shaking has phases: it basically has three-four levels of knocking. At park, A/C off, it doesn't knockas badly. At park, AC on, it knocks. At Drive, A/C off, it knocks a bit, At Drive, AC on it knocks hardest.

The check engine light is on constant, but was when purchased. During the shakes it sometimes flashes.

HELP!