Hi,
I'm sorry if this is wrong, or a stupid post, or even if you've got it fixed but I'll post it on the off chance that it helps you,
My 8v MK2 starter motor does similar when I start it sometimes, it seems to be a loose cable on the end of it (the blade/bayonet cable bit) it keeps shaking loose. If I take it off and plug it on again it starts first time. I really need to get it fixed. I'll try and get a photo of the cable later and show you if you need.
I'm not very good with mechanics and parts etc, it was the AA guy that showed me it and I haven't got round to getting it fixed yet, but hopefully it might help you,