jcb backhoe model1700b tranny works a little then nothing?

We have a backhoe JCB AND IT DRIVES FOR ABOUT 5 min. and then nothing. We took the solonoids on top of the tranny(actually the box on top of the tranny) and checked to see if the connectors were burnt or corroded. Not any problems there as it appeared. I was wondering if the key switch which activates the transmission could be suspect.
I hope someone has great words of enlightenment. Thanks, Dave.