Yep, I would be looking at the fitting that goes thru the transom for a build up of corrosion restricting the flow.

I looked this thread over quick but you said you replaced the impeller, Did you replace only the impeller? If you did, and it sounds like you are going to put another one in. I would put a kit in and replace the whole housing with seals and take a look at the shaft where it seals it could be blowing out the top of the pump. Make sure that seal that slides down the shaft is in place.

UPDATE: I gave up and took it to a real mechanic. He said he found NO ISSUES! IR temp readings on risers, t-stat housing - all fine. He said water was flowing through. Something clogging it must have broken free... I plan to "sea trial" it Friday after work.