you probably know this answer, but if not I will post it, it's not a complex answer, but the most common reason the engine will move out of time is caused by wear on the guides, which increases the slack amounts on the chain(s). it can cause the chain / sprockets to jump teeth
I believe the service interval for the timing chain from VW for the 3.2 24v vr6 is @ 120,000 miles, but most R32 owners do it by 100k.
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