I don't believe it's the safety switch in the seat, if it was the mower would shut down entirely not just the pto. There should be a PTO safety switch and it probably isn't set correctly. If you're trying to mow in reverse the pto will shut down automatically and engage when you resume forward. I had the same problem with both my CC ztr mowers and I just disconnected the switches and used a jumper wires and solved the problem, I wouldn't recommend doing that unless the unit is out of warranty however. If it's a new unit take it back to the shop and have them check the setting on the Pto switch or have the pto switch replaced.