2003 Buick Century Belt Squeek?

3.1L with air. When going up hills there was a belt squeel and power robbing feeling. I changeded the serpentine belt and tensioner. The squeek and power robbing effect is still there under load conditions. I checked the pullys on the alternator, power steering, air conditioning. All spin freely. The main pully off the engine seems tight but it is attached to the motor of course. Any thoughts??? Thank you!!!

Your A/C compressor could be on its last legs and starting to seize up. does the compressor rotate frely with the belt off? My wife’s Ford had a similar problem, and the car kept stalling until we cut A/C the drive belt.