Does it happen all the time while in boost for a period of time?
You may want to take a stab at burping the system. It could be possible you have a bubble or two stuck in the system. It is an easy way to rule things out as well; especially if you had a leak a while back as well.
Or it is possible that there is some lift happening at the heads while in boost and causing air to enter the system.
Your temp should be staying around the same position once you are warmed up.
I'd burp the coolant system first and see if you can reproduce it. Hopefully you have a pocket of air and that resolves everything.