[wizard]

Sounds like the thermostat is doing what it is supposed to do as far as opening. I suspect that the thermostat does not have any bypass openings in it so it waits til it comes up to temp, then opens. This will now allow water to flow and as a result you will see a short term temp drop until the system catches up. As far as the 210 goes, it sounds to me like it may be a 210 thermstat, very common these days for new vehicles. I say that because you get the temp drop at 210. If it were a 180, you would see that drop at 180. Did you check the thermstat before you installed it? The temp rating will be stamped somewhere on the unit.
You also said the temp fluctuates, but then only mentioned 1 cycle. Is this the case or does it go up and down repeatedly?
(I have finally solved my own cooling issues and will be posting a lengthy post in the 460 forum over the week-end.
West Coast owners should be interested.)
THIS HOWEVER DOES NOT MAKE ME AN EXPERT!!