Global Variable (timer) value changes when changing slides

I've created a timer with variables and layers that sits on the slide master, so that it will play persistently across all slides. When I jump from page to page, the textbox displaying the timer briefly shows a different number set, and then reverts to the correct time based on the variables. 

I'm curious as to what is causing this and if I can avoid it, or if I've made a simple mistake and just can't spot it.

Thanks for any help!