i upgraded to mirc 6.17 from 6.16 and now when its open for a while in the task manager i see 3 mirc.exe processes listed and combined they use 100% cpu. i think this is a mirc bug itself as it happened even when i didnt have my addon script loaded.
i have experienced 100% cpu load aswell after the upgrade a few times (3). after a close down mirc a process will still be there taking up most of the cpu time.
Ive had mIRC running for 1w 4d 19h 46m 22s and have had no issue. No major cpu usage and trust me I would notice im sitting on a 498mhz pos. The most ive seen mIRC use is around 70% and that was during a botnet attack, which I get the client connection notice from the server.