How long have you been running mIRC for? Even running it for 20 odd hours I don't notice anything unusual, I haven't seen my mIRC go above 11mb and it doesn't steadily increase or decrease, it might be using 6MB one second and then 30 minutes later using 11MB, and back down to 9, etc..
I log both channels and chats and due to the fact I'm on 3 networks and in about 11 channels I also get a number of PMs. I rarely clean out my logs either, one is over 40MB and another is just under 20MB - logs should not be an issue.
I have no timers running, infact I have very few scripts, only two of them go over 50 lines. The only thing I use that even resembles a timer is /pvoice with a delay.
There have been some complaints in the past of mIRC using a lot of memory, perhaps the search feature will turn something up, but whatever the case, I don't think it's an actual mIRC bug.
Try installing a fresh mIRC in a clean directory and run that for say 2-3 days (or however long it normally takes you to notice this large increase of usage). See if a fresh mIRC does it. It can be difficult to use a fresh mIRC when you're so used to how even small little snippet scripts act, but it helps determine the source of the issue.
Regards,
