1994 civic - engine revving itself in neutral

Hi all,
My prize 1994 Civic EX has a new problem, wonder if anyone knows what the prob may be...
In park and neutral, the car revs every 3-4 seconds. i.e. it's a cycle that
repeats itself every 3-4 seconds where the engine revs up slightly, settles back down, and repeats.
Otherwise, the car runs fine. Any ideas on what to look at?
Thanks.
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When did this start? What (if any) work was done on the motor prior to onset?
How's the coolant level?
How old is the thermostat?
When were the cap/rotor/wires/plugs changed last?
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TeGGeR

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TeGGeR, thanks for the reply.

No work done beforehand other than an oil change, the revving started in the winter, forgot about it, just started again. Happens on engine start, not on a warm engine.

Fine.
I don't remember replacing it, so probably 12 years. :)

I don't get tune ups often, so at least a couple years.
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after that would be a good investment. The coolant level in the radiator can be right to the top but enough air can be trapped to cause your symptoms.
Mike
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