rough idle...help!

i have a weird problem
my 1993 pontiac grand am sometimes
idles rough
meaning, when its in park, it will sometimes rev up and then down and up and then down again
nobody seems to know what the problem is
ive had opinions say it needed a tuneup, a new
intake manifold gasket, transmission service, and to no avail
there is no idle screw on this car
it rough idles very infrequently, and the mechanics cant diagnose it because they have yet to see/hear it
i have had all the above done to my car, as well as a new TCC solenoid,
and nothing seems to work.
it does no harm, because it only does this when in park
as soon as you shift it into anothe rgear, it stops and idles regularly
any ideas?
thanks a lot

quick question, does the gear indicator light look dull of go off when the gear shifter is in the "P" position? If its is then its the "Gear indicator module" Basically the computer doesn't know what gear its in so it keeps messing with the idle air/fuel mixtures. About a $40 part, easy fix once you get the part.

If fixing the Gear Module doesn't work, then I would suggest selling the car. The year of that car is not good, I would recommend selling it and using that money as a down payment on a newer modle.. maybe a '99 Grand AM. I don't suggest going higher than a 2002 on the Grand AM's, they seem to not have ideas for what causes problems for them yet. A '99 is sufficient.:sunglasse

GM had the most problems with the 99s of the 99-2002 models , common for first year models too have problems that still need to get worked out. So I suggest a 2000-2002 with a good service record and no body damage and you should be ok.

no need to sell your car :rolleyes: its a $40 repair.

thanks
you were right, the light does go out when it idles bad
so the gear module must be bad
is it an easy part to replace, or should
i bring it to a garage?

I have the same problem, let me know if changing of the gear control module worked, I tried cleaning throtile body, completely including IAC sensor, changed sparkplugs and cables just to be sure, but still same thing. My best guess is that one or more of the injectors is cloged up but then I would feel the same thing in D, or maybe some vacume leak. Try inspecting all vacum lines.