Just finished installing my new heater core on friday. Drove the car all weekend. On sunday the car started acting up. Got it home and let it sit in the driveway at idle. When I put any of the HVAC controls on the car drops then rises in RPM's, finally after a few times of putting it on and off the car died. Sounded like some kind of vacum leak but im not to sure about what it could be. I checked all my hose connections going into the heater core and out of it and eveything seems to be good, no leaks. Any suggestions?