150 mercury overheat

[cambridgedojo]

I purchased a 1987 mercury 150 OB oil injected. The motor and boat had not been in the water for 5 years. Before the initial trial, I had a mechanic replace the impellar, check compression, etc. The engine overheats at high rpms but when brought to an idle, the gauge comes back to 1/4. The mechanic has replaced both thermostats and cleaned water jackets. The tell tale stream is great at all rpms. When overheat alarm comes on, as soon as I can get boat stopped and feel the water stream, it is hot but not unbearable. Is this a cooling problem or could it be electrical? <br />Thanks