Vibration at Idle

Hello,

92 Honda Civic DX 5sp

Whenever I'm at a stop the RPMs are lowered (don't know how low since the
civic doesn't come with a tach) and the car starts vibrating really bad. The
frequency is low and "bassy" from the inside. Pressure builds up in my ears
from the low frequency. Any idea what it could be?

One thing I noticed is that whenever I turn on the headlights, the RPMs go
even lower and the vibration becomes even worse; when I turn them off, it
gets slightly better but noticable at the same time. Ideas as to the
problem?

I noticed there was a ground wire going from the chasis to the transmission
that is almost cut in half due to strands of copper being cut. I'll replace
this ground wire tomorrow and also replace the engine to chasis ground to
see if that makes any difference at all.

I don't know if it's all electrical but it seems that some of electronics
are playing part on this low rpm/vibration thing.

Any ideas/suggestions are welcome!

TIA!

John