Mercruiser question - Temperature problem

[abl1111]

I have a 2003, 350 freshwater cooled motor w/ 280 hours - still under extended warranty. Bravo 3 drive and cooled via SouthBay strainer.

Temp gauge went to HIGH a few weeks back. Stopped motor, called mechanic - he said if no alarm - no problem. Motor runs fine. I run usually at 3200 rpm's...

Coolant was 3/4 gal low. Oil, was 3/4 quart low. Added both - and changed the temp sending unit and the gauge.

Sometimes, she will run perfectly but at some point within 30 minutes, she will read High.

Used infrared at thermostat/sending unit - at 165'ish degrees @ idle. Bypassed harness via a jumper from sender to gauge - still HIGH.

Ran cold hose water over sending unit, temp. gauge lowered a bit from High - then rose when water removed.

I'm all over the place with troubleshooting; bad thermostat, check compression, head gasket, pressure test cooling system, manifold blockage, etc..

What are your thoughts ? Thx