98 Crown Vic LX engine surge

Hello everyone,I looked through the threads but couldn't find anything on this issue.I have a 1998 Crown Vic Lx with 66,000 miles on it.We bought the car about 2 years ago and at that time it had only 32,000 original miles and looked like it had just rolled off the showroom floor.The car was well maintained and we have kept it that way.Over the last few months we have started to have a problem with the engine surging at idle when the Heating/air controls are in use.It's like the engine tries to die and then comes back up and it happens about every 5 seconds.It stops when the control is cut off.The fan speed control has no effect,just the main control.Anybody have any ideas?

vacuum leak?

Check the Idle Air Control valve. Sounds like that may be part of the issue.