My bet is the impeller, those things need a couple runs, on them, to soften up.....
One thing, have you made any changes over winter, to kill time LOL? I don't know which engine it is, middle, or outside, but if you made NO changes, I bet on the impeller!
You need to run washers, where the thermostats would go, that water has to be slowed down to cool the heads!
Hope this helps..