Randomly dropping target

Hi,

I have been playing on the Icecrown with my friend and it has been great so far but there is an issue which only affects him. Sometimes during a fight my friend will drop his target at random, after casting 2-4 spells, we checked for sticky targeting, hardware cursor, didn't help at all.

I didn't see anyone else talking about the solutiom for the problem here, did someome stumble upon this issue in the past?

It may be worth noting that my friend's game is capped at 60fps too for some reason and the only refreshing option is 50hz