I have a 1990 or 1991 John Deere 160 Lawn Tractor with a 38 in mowing deck. When I go to mow the blades will suddenly disengage. The mower will continue to run, the blades will not cut. I have put on a new pto clutch, pto switch, and safety switch and none of these have worked. I have checked the spindles to see if they heat up, but they do not. Any ideas on what is wrong? Could it be the battery is not giving enough juice because of the new pto clutch?