There is a thread just two above yours with exactly the same subject! See
this thread as it has almost every suggestion people have thought of.
Marantz: mIRC was never designed to handle files that you'd need speeds of 120kb/s to "download", so it's unsurprising you're running into problems. I usually find that the majority of people that have connection issues are doing either 1) Lots of DCC "downloads" at the same time or 2) Lots of very fast DCC upload/downloads. mIRC is a chat program, the DCC protocol was designed for the exchange of small - normal size files. So 5 MB shouldn't be so bad - 300MB is.
You might also want to use the Forum's
Search feature as lots of people have started separate threads about the issue rather than posting in the sticky, and have come up with their own solutions and ideas about the issue

G'luck!
Regards,