For some time my daughter's 1985 Civic SI has had an intermittent idle
problem, and now it has become much worse, to the point that it can't be
ignored.  At idle the engine speed cycles up and down between about 1000
rpm and 2000 rpm, about twice a second.  It is almost as if the engine
cuts out when the revs get up to 2000, then cuts in again when it drops
below 1000 rpm.  Now the condition seems to have become permanent (funny
how problems like this never seem to get better) and it affects low
speed driving, e.g. in heavy traffic.  While driving if the revs are
about 2000 with little throttle the engine does the same cycling, making
driving very difficult (once was enough for me to convince myself not to
do it again).

Perhaps someone can tell me what is likely to be the problem.  Does
something need to be cleaned, or is it a matter of replacing a sensor or
valve?  Otherwise this car still runs very well - very sporty - and I'd
hate to junk it.  Thanks in advance for all suggestions.

Gib