My old Craftsman ZTS7500 lopes or lunges at low speed under a load. Does OK at full speed. When it first started doing this I found the transmission oil low on both. I refilled them and it seemed to help but started doing the same thing after a while. Trans still full, no leeks. I there an adjustment for this? Mower has over 1700 hrs on it and was probably not meant to last this long. I take very good care of all my machinery. The trans is sealed and suppose to be maintenance free. Their is a small breather hose on the trans. Oil can be replenished there. Anyone have any suggestions? Thanks