First thing i would try is to test on a clean copy of mIRC without invision to see if it still happens.
I would guess you are getting disconnected with 10053 (software caused blahblah) error? Use the search feature here, all forums, all posts, using 10053 and you can see some of the things that seem to be causing this for xp users. Doesnt seem to be any one thing fixes all solution.
