Last fall I had a major service done by my local authorized Merc mechanic - he did whatever is involved including tune-up, filters, plugs, lube, and did the water pump and thermostat. This spring I noticed although the engine starts/runs fine, when I get up to say just before planing speed (about 1500-2000 rpms, probaby about 15-18 mph) the engine wants to overheat - the temp on dash gauge jumps from an idle temp of about 150's up to mid to upper 170s, then the Smartcraft system shuts the engine down and the warning comes on the gauge to reduce engine speed. When I idle back down to say trolling speed (5-6mph) the temp drops - plenty of water comes from the rear indicator spout on back of motor. The other odd thing is if I just gun it and jump from idle up to plane, say up to 3000 rpms or more, 25-30mph, then the temp drops way down to about 110-115. So it appears to be getting plenty of water forced in at speed. At slow idle forward, as I mentioned, it doesn't overheat but it does get up to the 150's which I have heard is normal and no problem. I just don't get this overheating at mid-range - its when your bow starts coming up out of the water and the boats on more or less a sharp angle, upward, just prior to the boat planing off level. If I keep it at that mid-range speed for say 5 minutes or more, with bow up high out of water, then it wants to overheat & shut down. Has anyone had experience with this problem on the Mercury Optimax 2-strokes or have any idea what it could be? I did ask the mechanic - he said the following, which for all I know may be true, but it sounded a bit far-out - he said "sometimes when you replace a thermostat on these motors, it traps an air pocket or air bubble, up under the thermostat area - he then said, just gun the motor from slow idle to wide open a few times and it should knock that air bubble/pocket out of the cooling channel. Just repeating what he said - I have no idea if its true or just some kind of horse-hockey he is telling me - any help is appreciated -

Not an air bubble,
It is most likely a stuck poppet valve, but if you changed your impeller last fall, I would change it again, the impeller on the 2.0, 2.4 and 2.5L motors Opti or not, need impellers changed every 12 months, More so when they don't get used much