Droppping Drive Belt on Simplicity Broadmoor 16v Hydro

I have a Broadmoor that will drop the drive belt off the pulley on the transaxle if it's under load, such as starting fast, going up any hill, or pulling a trailer. I replaced the original belt with an automotive fan belt because there is no dealer near me but the problem is still there. Do you think the belt is still coming off because I didn't use the OEM style belt from the dealer or because something else is wrong? I have heard that the automotive style belt can stretch, allowing it to come off the pulley. I can't find any other problems.

Thanks for your help,
Justin