Definitely (from my admittedly limited knowledge) sounds like a power VIS motor thing.
You suggest you've tested it, but given that it's intermittent, I'd say the test is not conclusive, as it may be beginning to fail/struggle.
I'd send "Stocktake" a PM and ask his thoughts, myself.
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[SIGPIC][/SIGPIC]
Villa, Stella, Balti.
It's Brummie latin that means, er......
Vis tester available to local V6 owners 
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