overheating question, stumped!

[dbetterred]

I got a new (rebuilt) 150 merc installed this spring. I've been having overheating problems since day one. The motor guy is fixing the problem but, he is getting stumped and i'm getting frustrated.
The motor overheats between about 1300 and 3000 rpm. The pisser works the whole time, until it get too hot and its just steam. Above about 3000 rpm it will cool back down. Below 1300 it will run all day.
It only has the problem while in gear with the boat moving.
So far, everything has been replaced. There is a new powerhead (after the original). New poppet valve. New Thermostats (also tried without). 2 new impeller and housings. Even a different midsection and water tube. Literally everything except the lower unit housing has been replaced.
The boat is at the motor guy right now and he is getting a lower unit for it. I hope to have it for the weekend.
The height of the motor is the same as my old one so that shouldn't be an issue.

Other than overheating the motor runs great.
My motor guy seems knowledgeable and is doing his best to make sure I'm satisfied. I'm just getting frustrated.

Any ideas?

Sorry 'bout the long post.