Actually, rough is an understatement. I inherited the Stang from my late husband and he had not had it long when he died. It was working fine until last week when it started shaking badly, even while idling. I poked around and found the vacuum line wasn't plugged in. When I plugged it back, the car smoothed out and worked great...for 1 1/2 days. It started again, and we could find nothing out of place this time. While we stood there scratching our heads, it smoothed out on it's own. Yesterday I drove it to town and it started all kinds of shaking, shuddering and now a kind of clanking noise coming out from under the hood. The engine shakes badly enough that if the hood is closed, it jumps up and down and the engine itself looks as if it is trying to twist out of there. Every now and then, for about 5-10 seconds, it will sound fine again. Could the day or so that the vacuum line wasn't connected have damaged something? Any ideas what it could be? Thanks in advance.

Maybe broken engine mount(s)?

Maybe broken engine mount(s)?
That was my first thought, about the engine mounts, but have checked them and they are ok. Thanks for replying though.

Hey , You Might Check O-2 Sensors, Air Filter, Plug Wires, And IF ITS GOT ONE The crank Shaft Possisioning Sensor.

The problem turned out to be the harmonic balancer. :)

Hey , You Might Check O-2 Sensors, Air Filter, Plug Wires, And IF ITS GOT ONE The crank Shaft Possisioning Sensor.
These car's don't have them.

Glad to hear you found your problem. I was just going to suggest engine harmonics or timing.