Loss of movement forward or back

Hi, I have a 1999 763 high flow- while moving alot of sand yesterday I lost movement forward and backward only when I used the machine at full throttle- the levers were almost impossible to move- it felt like they had heavy springs on them. The machine wasnt overheating - it actually was running fine- if I backed off on the throttle it would work fine- I only went to full throttle to climb over a sand hill- the machine has ott on it (rubber tracks). I went to move it last night and it worked fine at full throttle (I was just seeing if that problem was still there). The machine has 1291 hours on it. Does anybody have any ideas what might be wrong or where I should look? I will be using it again this morning to finish moving sand.I joined this forum a while ago and although I really dont post on it , I do look at it every day and you guys are extremely smart .Thanks for all help...Chris