The RPMs are raising and lowering when idling once the vehicle is warmed up.
This might be due to a vacuum leak. Vacuum leaks are common in older vehicles because of the age of rubber lines, gaskets, etc. The vacuum leaks alter the air fuel mixture and as the engine tries to adjust it, and the idle will tend to be erratic. A mechanic can help pin point the source of the leak and fix the rough idle.
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