Surging during idle

[fxgovers]

Car: 2001 Boxster S

Problem: When idling the RPMs go up and down from 700 to 900 and back again in about a 3 second cycle. If I turn on the A/C the idle settles down and is steady. Only occurs with the A/C off.

I hooked up a ODBII scanner and get no codes from the engine at all.

My mechanic thinks its a vacuum leak - any suggestions?

Thanks,
Francis Govers
Nashua NH

2001 Boxster S Polar Silver/ Black int.