Had a similar issue with my 755 when I first bought it. Have you taken a close look at how the PTO lever functions yet? If you haven't take a look at it closely, it is actually two pieces coupled together by an electromagnet. When one of the safety switches ( I think there is two but I could be mistaken) trips, the magnet looses power the the half of the lever that is connected to the PTO falls back to the off position.
In my case I could sometimes ease the lever up and it would hold, but in winter on a cold day things would be too stiff to engage and it's hard to blow snow without a PTO.
I pulled the magnet out and bench tested it with a battery charger and everything was good, the terminals needed a cleaning and it's been working fine ever since.
The safety switches are known problem areas and if you suspect one of the them, pull the electrical connector off and check it with an ohm meter to ensure it's opening and closing correctly. There is also a control module that that provides a brief delay (for those times you go over a bump and the seat switch trips) but I think that part it pretty reliable.
Hopefully that gives you a couple ideas on where to start looking.
Jason
