OK, so I remove the Frost strike button from the sequence and spam it when I get enough RP (as I went to bed last night after posting this I realised that was what was causing the problem).

@Stealthy - I had wanted this to only be run once during combat. As I have the glyph of disease, each time I cast pestilance, the timer on my diseases is refreshed. As such even if I switch target I wouldn't need to do frost strike or plague strike again. I'm hoping for this to be able to maintain AOE aggro - this is something I've been struggling with. To gear up the tank so I can start soloing instances, I've been doing randoms with pugs and find that I'm constantly losing aggro with either this or an unholy spec.