My initial testing on Alpha runs with the GT-31 produced some pretty strange speed graphs for the Alpha's, especially with trackpoints. (Which was disappointing as with the GT-11 the trackpoints seem to produce the more realistic results)
I had no evidence for an explanation until now but we thought it would be to do with the sirf3 chips internal filtering.
Testing on the weekend with one GT-31 with a new firmware that allows 'smoothing' to be turned OFF in the chip shows our hunch was correct.
With the smoothing turned off the results from the GT-31 were very consistent with the trackpoint results from the GT-11 also worn at the time and don't show the strange artifacts that the GT-31 with the smoothing ON shows. See this screenshot of two GT-31's side by side. One with smoothing turned off (Top):