Idel changes and stalling when heater is on
I have a 2k Taurus SEL DOHC. With the heater on, the idle increases a few RPM's about every 20 seconds or so then returns to normal. This cycle repeats as long as the heater was on. Does the same when driving, and now when I come to a stop, the car wants to, and sometimes does stall. When heater was off, idle problem went away, but now, even when heater is off and stopped at light for example, engine seems to want to stall more often.
I think I can hear a vacume leak, sounds like hissing noise, but can't seem to locate where it's coming from. if anyone else has experienced this problem, could you point me to the proper place/part that needs fixing.
Thank you for any assistance you could offer