2001 suburban

First things first, Hello to everyone and its good to come back to this most informative site.
I'm in need of some information on my 2001 sub, Whenever the a/c - or heating system is turned on, sitting idle the engine rpm's fluxuates from 500-to-zero and back,and as soon as I shut the a/c / heat system off,the rpm's stop fluxuating.Engine goes back to normal rpm.Though this doesn't happen all the time,it happens.

thanks to all,
sub 2001