I never solved it (well i did actually, i bought a p2m).
For whatever reasons the powertap used to throw huge power spikes and i'm guessing here, because it thought i was chucking out a few gigawatts, it would correct speed etc to match that.
I changed to a brand new laptop, so it wasn't like it was struggling to run GC.
Even after going to a G3 hub, the problems remained, with or without a speed/cadence sensor.
The P2m performs flawlessly on GC, no spikes, or dropouts, so i can only assume its something thats missing in the programme. In fact the only problem i had was a decathlon sensor that was duff out of the box, causing problems. Binned that, and it works fine.
You could always edit the file to find anything above x watts, if you looked to see what you were doing before it spiked. Should only take a few mins per workout, but a bit of a pain nonetheless.
Official cafe stop tester