We have a John Deere X724 mower and after it runs awhile it seems to run a lot slower. Engine runs fine but it seems to slow way down. Would you suggest changing the hudraulic oil and filter? Mower is probably 5-6 years old and has 300 hours. Hydraulic oil looks OK and level is good. Mower is all-weel steer and otherwise we have been very happy with it. Any suggestions would be appreciated. gc