Quest Problem

Whenever i accept a group quest while 5 boxing everything seems to die. my computer which im running 4 accounts on will lock up completly. now my machine is definatly capable of running 4 instances fine, i get about 30 fps normally. For addons im using multiboxer and questhelper. Now im not sure if this has anything to do with the multiboxer addon or what it is but i figured this would be the place to ask. Another thing a noticed today is my quest log wouldnt update. I was in felwood killing Jadynar people, and nothing would update. If anyones ever experienced the same problems, could you please lend me some advice. Thanx again.