I have it and I have the exact same issue. As spdrcr5 alludes it is in fact due to frame flex. Basically when you are out of the saddle the frame is flexing under load and changing the distance between the magnet and sensor enough to intermittantly lose passes of the magnet (read a lower speed).
I have tried to remedy the problem by switching to stronger magnets, playing with the sensor arm location/setup, etc. My issue is that the design of my chainstays on my frame does not lend itself well to use of this computer.
Currently I am living with it. I only notice a small variation in distance over a century when compared to others that I ride with. The ill effects are not worth replaceing the unit with one of my many wired units.
BTW - I just discovered this last weekend that the issue also occurs when under severe climbing conditions even while seated. For the record I am using mine on a Giant TCR C2 from 2005.