As others have said. It's probably the engine to transmission belt. I had the same problem with my Snapper Pro belt drive last year. I knew it couldn't have been the drive belts because they were replaced the year prior about mid-way through the season.
I pulled the transmission belt off and took it to Napa and they were able to find an exact match for significantly less than the mower shop down the road (which had to special order it). The mower shop actually told me to check Napa since I needed one ASAP.
Also, I've noticed mine will slip from time to time if it gets wet.