Cadillac Repair: misfiring, spark plug wires, spark plug wire

Question
I have a problem that is intermittent but has me puzzled. I changed the fuel filter and the car(91 Deville) ran smooth as long as I didn't turn on the heater. When I turn the heater on it runs rough at times and then it doesn't. It backfired today when I increased my RPM going up a grade. Last summer the A/C wouldn't cycle. Could it be a problem with the climate control panel?  

Answer
Could it be a problem with the climate control panel? NO !!

Could be:

an intermittent fuel injector.
An intermittent spark plug wire.
An intermittent spark plug.
Make sure none of the spark plug wires are laying against the heater hoses or any metallic surface.

when was the last time you replaced the spark plugs and the spark plug wires ?.