5200 PTO - stops/slows

I have a 5200 and recently have a problem with the PTO slowing to essentially off intermittently. A friend just used it and said that he had to put tractor in granny gear for PTO to work (properly) any suggestions on fault? Also, just change oil and filter and ran tractor hard for 16-18 hours...later found diesel in crank case. JD dealer says seal on pump needs replaced. Anyone else have any ideas or have had either of these problems?

Thanks,