Sounds odd! My driveshafts have never missed a beat.
The only thing I can think:
What's is the wear on the driveshafts like, are the 2mm holes round? Or have they started to go oval? Do the pins sit in the holes correctly?
I there is any movement here, that could cause the screw to loosen. Everytime you hit the power the pin could get a sharp shock causing the grub screw to loosen.
A good tip is to put some tipex on the flat, that way you can ensure the flat is under the grub screw.
Also, get some new grub screws, then get a dremel/file on the ends, get as much surface are as possible.
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