I have been wrestling with this issue all day. Here's a couple things:

1) I just got a new router, a D-Link DIR-601, and I began getting 10053 errors exactly every 8 minutes.
2) I've been using mIRC since 2002 without every seeing this error once! laugh
3) It is OS-independent; I've duplicated the error on XP and Win7. It is also server-independent. I've used a dozen different IRC servers to prove this.
4) This error only affects mIRC. I have used XChat and a couple browser-based IRC clients; none are affected. No other programs I am running are affected. Solid connections for games, music streaming, video, you name it. ONLY mIRC is affected.
5) I have tried UPnP on and off, changing mIRC ports, randomizing them, disabling firewalls and antivirus. Nothing works. This is, in my opinion, definitely a conflict with my new D-Link router.

So... that said... I'm gonna keep plugging away at it until I figure it out. If and when I do, I'll post here.