Chevrolet: idle with a/c on, windshield defroster, chevy blazer

Question
 i have a 2001 chevy blazer zr2. recently i noticed that my idle kept jumping like when your plug wires are arching out. idle kept dropping like it was gonna stall. i was gonna change the plug wires but then i noticed it only does it when i put the a/c on or the windshield defroster which works with the a/c. can you please tell me what the problem might be. this started happening a few weeks after i had the heater core replaced.

Answer
HI Mike,
      I think that it could be a relay, you might have to get it hooked up to a comp. to see where it is comming from, but try to listen for a clicking sound from the fuse box or under the dash when you find the one thats clicking try to get a new one and see what happens ok.
Roger