idling like a tank...

I have a 1991 Mercury Tracer 1.9. I t has 86K miles and it sat in a garage from spring '92 until '98. ABout 2 months ago, the engine has been going crazy, like it is going to jump out of its compartment. The whole car will shake and rumble as if I'm driving down a road full of potholes at top speed, even while idling at a red light. This happens about once a week, and sometimes I can stop it just by turning off the ignition and restarting it. A couple of times, the check engine light came on, just breifly, and then shut back off. What is going on?