Jlg 450 series 2 Drive problem

I am working on A JLG 450 series 2 left with a drive problem starting out from cold you can drive 600 to 800 feet and the machine slows to a stop you try and engage drive reverse or forward it flinches but will not move the boom controls work fine I did a hydraulic pressure test when hot and it came out within specifications anyone who could give an opinion or help it would be greatly appreciated thank you Dane in NH