Case 1835b uniloader wont govern properly

Hi all ,I have an old case 1835b skidsteer that runs perfectly for about half an hour then the hand throttle cant be set and the machine cant be turned off ( has electric shutoff ) just seems to rev at about half revs an doesn't govern properly. I have pulled the fuel pump apart and cant find any obvious problem .It has a stanadyne DB2 pump and it has the solid weight retainer so the flex ring is not the problem. I am left scratching my head asto the problem any help appreciated. Cheers, Terry