Oak - it's a known problem with the 910 specifically. Mine does the same thing. My husband and I would run together, me wearing my 910 and him wearing the 305 and his pace was reading steady (reasonably) and mine would jump around wildly even though we were side by side.

I hope they resolve it, but I've been using avg lap pace for my own needs right now. Well, before I stopped being able to run at all, anyway.