Originally Posted by Powerdown
OK, I've tried something different today - I've locked my phone in so that it can only use 2G rather than 3G. It's still dropping calls occasionally, but it's not locking up and shutting down anymore (touch wood).
Whether this is co-incidence I don't know - I'm just experimenting having read advice on other forums with problems as the device switches between 2G and 3G. I'll keep you updated.
I was going to suggest the same about 2g & 3G mode. Also check your Auto On/Off options and Disable them.