JLG 450 Y2001 Swing/turntable problem

I bought a second hand JLG 450 Y2001 4000h but in good condition. After fixing several electric and hydraulic problems the machine worked nice but unfortunately the swing runs irregular 'bumpy' at all angles and may occasionally even lock (!) abruptly during the turn at any angle. The same effect as a broken worm which lock into the gearteeth. This effect is much more when swing activated from the ground control and no load in the platform. Swing activated from the platform (load 90kg) is better (more balanced?) However there is very little play in the swing gear/worm; the outer swinggear looks undamaged and not worn at any teeth. The bearings on both sides of the worm are ok. I did not find metal parts or pellets in the worm grease. Page 2-12 turntable assembly: https://csapps.jlg.com/OnlineManual...0A_AJ/Parts_3120750_01-28-11_ANSI_English.pdf I tried to disassemble the worm for inspection but I can't get it out of the housing far enough . Left side bearing and seal removed (not motor side). Is it possible to remove the worm shaft without disassembling the complete turntable gear? I guess not because the end of the shaft is too big to pass the gearteeth.... When the worm is disassembled half, it is possible to swing the boom 10-20° by hand which feels smoothly and easily, no bumps. So I think the bearing is ok but it's only possible to test at a small angle. The turntable gear bearing has been greased at several angles and the worm was full of grease. The outer gear has been greased as well. Makes no difference. Turning the worm manually with the outside bolt (closed assembly) is very hard, even when the machine is on a flat level. May be that's normal, I don't know the force needed to turn manual. I think the reason is a damaged worm but I do not find traces on the outer swing gear teeth. What else could be the problem? I hope and do not think it is the main bearing itself. I do not have the lift equipment to remove a complete turntable. If I can't fix it with standard tools then my investment is old iron... Thank you Wilfred