By 'download' do you mean mIRC's DCC function or downloading off of the Internet via something like Internet Explorer?
Do you have any "speed patches" which are supposed to speed up DCC?
Some ISPs limit the amount of bandwidth you can use, this problem has been reported before. Contact your ISP to see if they do.
Make sure mIRC is the only running program, you might be using other programs which are using up a lot of resources on your computer.
Again, if you use the search feature as I suggested before, you will be able to find threads referring to mIRC freezing, then unfreezing, then freezing etc etc.
G'luck!
Regards,