On the old schwing pumps that i've driven it would jump out of pump gear when the truck would runn out of air. If I tried to blow a ball thrue a long pipe line and suddenly it was like GRRRRRRRRRRRRRRR from the gearbox ( sorry for my refined technical english).. Could this be the case at your pump?

That could be his problem cause it does the change over with air, he could have a leaking valve.

Is it mounted on a mack ? Ive had this problem before and it was a transmission problem not a pto issue, it did quickly become a pto issue but the cause of the problem was in the transmission.