If it is not the belts then look at the crank pulley .......it is two pieces and once the rubber damping material in between gets old it can spin at less rpm on the outer half of the pulley, where the alternator belts are then as the engine rpm slows the outer half will again turn at engine rpm. test this by settingthe cam pulley to it's timing mark and see if the mark on crank also says 0 degrees. if not the crank pulley is bad. good luck to you!