Garmin 305 issues

Now this is an interesting one. I wrote a while ago that I was having a couple of issues with my trusty Garmin 305 Forerunner. It appeared to be switching off on me during runs and races.
I narrowed this down to the fact it was switching off when I jumped over a stile or a wall or something, figuring that I was knocking a button or something like that.

I realised that this wasn't actually the case, as it kept turning off recently when I just pressed the mode button.
Odd.
So I uploaded all the run files and information on to the computer and did a soft reset, went for a run- it did the same thing.
Ok.
Hard reset, try again.
It worked ok for 2 runs, and then switched off on me seemingly at random.

I've now looked at what is happening and can't work out for the life of me WHY it is happening- let me explain.
Watch on left wrist. Happily ticking away.
I can wave it around, I can run normally with it, and can do all kinds of things, but I cannot bend my wrist without it switching off.
Once I have the watch on my wrist and I extend it backward, as if I am putting my hand down to hold onto something- like a stile, it switches off, immediately and without ceremony, without beeping, nothing, it just switches off.
I can't replicate it when the watch is off my wrist, but can when it is on.

Hopefully the video I've embedded will show that- none of the buttons are pressed, nothing really happens except that the unit switches off.
Anyone got any thoughts? Its served me well for a while, and this really does seem like a technical fault.
Garmin? Got anything to add?