In reply to Alex Newton:
I get a similar problem sometimes, particularly when moving my body towards my thumb when on a sloper.
There was a stage I was getting it regularly on the thumb side of my wrist, it would lock out (or feel like it was at least) and i'd get quite a bit of pain. The biggest thing for me was changing my mouse in work (IT desk job) to a mouse that puts your wrist in a more natural position and avoiding using the touchpad thing on laptops overmuch. Some taping, flexor/extensor stretches and knowing when to back off and walk away from a problem/session when it feels dodgy helped as well. Now I don't need the taping and when I do have issues it generally coincides with laptop use.
As a caveat I did get it professionally looked at (X-Ray and MRI) and it came up clear, so if in doubt best to see the physio!