Have you raised the hood and looked at the belt while the A/C is on? You probably have the A/C on when you have the heat on and its putting a large load on the belt.
As you said, could possibly be a bad tensioner.
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Current Jeep: 2004 Jeep Liberty
-Atlantic Blue
-Air Ride Rear Suspension ; OME/Rustys Lift ; Powertrax Locker ; Moog Lower Ball-Joints ; 245/70/16 Yokohama Geolander A/T-S's on MOABS ; Flowmaster 50 SUV
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