1999 Civic EX 5spd
140k miles

At idle, such as at a red light, *sometimes*, maybe 1/3 of the time to
1/2 of the time, the tach will show the RPMS going up a few hundred,
then back down several hundred more. Up and down, up and down... Only
when the AC is on. Sometimes the RPMs have dropped significatly, and
the engine seems to shake as if starved for gas or air or whatever.

Vaccuum leak in the AC system?

PS... not that it matters, but the AC control unit is on the frizt...
this is the dealie next to the stereo, with the 3 knobs that control
a) Which Vent, B) Temperature, C) Fan Speed. During the fall, I
started having trouble with it, then in the winter all the fan speed
knob stopped working. Temps have warmed up with Spring at hand, it
works better now. I'll think about replacing it during the fall
unless it dies in the summer. Mentioning this...just in case....