ElvUI was one of the unit frame addons I tried out, though I eventually didn't go with it. I did use it through two 10-man raids and I didn't have any problems at all.

Could it be a lag problem? I notice sometimes with all 10 busy places and I am spamming keys to kill something, the keys may sometimes stop and I have to switch to every window then back and everything is working again, I dont have to completely exit. Should try that.

I attribute it to lag, but it happens too seldom to be able to experiment.