The tyre is moving under load hence not catching all the time.
5mm spacers should be adequate.
I ran 5mm flat spacers (non hubcentric) with standard bolts on the MK4 with no problems at all.
Are you running 195/50/15's if so very strange as mine were nowhere close.Might be worth checking your camber.
Long shot, but is the wheel bearing ok, I would check that as a urgent 1st step.