While bush hogging with my Zetor 5211 my PTO will randomly pop out of gear. It seems to do it on tight turns and anytime the grass gets really thick. After I stop, throttle down and put it back in gear (with clutch of course) it shakes very hard initially then smooths out. If I put my foot in front of the PTO shifter it seems to help. I've noticed it does it on the same spots in my pasture time and time again, usually tight turns or on inclines. I'm wondering if this is the tractor or the bush hog causing the problem. There is no forward/backwards play on the drive shaft. If I raise the bush hog a bit so it doesn't have as much resistance from the thick grass, it doesn't seem to do it.