Since it is still happening with a clean install of mIRC, we would need to know a little more to track down the issue.
It could be due to an incompatibility of mIRC 32bit with Windows 64bit, however there are other potential causes.
Did you use a clean mirc.ini or your existing mirc.ini when you tested the clean install of mIRC? The issue could be related to a specific setting in mIRC.
Are you using anti-virus software? Are you downloading files? This can often lead to temporary freezes due to virus scans.