I thought it might be an issue related to a script, which is why I didn't post it here (didn't want to assign blame to mIRC, as you tend to produce a quality product).
Glad to know I'm not the only one with the issue though.
I've downloaded the beta and have it running. Will report back if it crashes again in the next 48 hours or so.
I have noticed that when the channels are mostly idle, it doesn't happen as often, so will have to wait until at least one is active before it might crash.