ac pump, alternator or power stearing pump goin bad (making it hard to turn) could cause small loss of power and a slipping belt. pull the belt and see if its hard to turn any of them.

 

I would agree with Crabby. I dont think a bad belt would cause a loss of power. I would look at all the pullies. Remove the belts and spin the pullies by hand. Most likely the ac pulley.

PS: The gator belts work well