<VV> fan belt trouble

[Landis, Art MR]

I finally got my '63 110 hp/PG running.  She's not road worthy just yet but
I was encouraged that I got the engine running after 5 months.  I've had a
strange problem.  When I have the engine running for a while, the fan belt
will twist on it's own and the V-groove (ribbed side) of the belt will twist
away from the fly wheel.  The grooved edge will stay in the fan, generator
and adjustment pulleys correctly but the belt will twist incorrectly around
the fly wheel.  This will then cause the engine to stall.  This has broken
two belts now.  Any one else have this trouble?  Any fixes? 
 
Art