[quote="leo"]It seems like a very unusual problem...
#1 Maybe this guide to tracking down memory leaks will help.
[/quote]
Yes, I need to try that out soon.
My very thoughts 
I shall be trying those next...
[quote]
(Thanks for trying all these different things, and sorry we're having to try wild guesses to work out what's going on.)[/quote]
No problem I do this stuff for a living, I know how difficult it can be to diagnose a problem by remote!