All day yesterday I had no problems but this morning everything was fine...until I went onto the ebay website. If I have ebay and mIRC open everything goes as described in the OP. As soon as I close ebay it reverts back to normal. I've been able to recreate it every time I've opened ebay and so far it's the only thing I can connect to the problem.
I guess the short answer is don't go on ebay!
Also, I checked the log files whilst mIRC was playing up and they all appear as they should be - with the right chat in the right channels and in chronological order.
