Vastly inconsistent power reading from outdoor vs turbo with Vector 3's

I'm at a loss, Vector 3 power reading when on the road is vastly different from when on the turbo using the same bike linked with Forerunner 920.  The reading is correct when outside and using GPS but it goes crazy high when I hook the bike onto my bkool turbo.  I even tried setting the watch on gps instead of 'indoor ride' on the turbo and it still goes off the scale.  I really am at loss why it's doing this and is preventing me from doing an FTP test so I can work out my zones...  which in turn defeats the object of me buying power pedals in the first place!  Any ideas gratefully received.  I always connect and calibrate! Is there another simple step I am missing?