2001 suburban

[sub2001]

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