No, it is (unfortunately) just a standard cooling system (it is only
17.5' boat). Simply put, recently the boat starts hot (needle creeps
to the 200F to 250F range) and after a random period of time (averaging
1 to 4 minutes) needle suddenly drops to around 150F and all is well
and engine appears to stay at approx. 150F.

If it was the impeller or clogged hoses, wouldn't the problem continues
after 3-4 minutes of running? Andre