hi,
been offline for a couple days, may be stating the obvious but ,our engine,s started to overheat when under load,ie @over 3000 rpm ,ours turned out to be blocked weed filters ,you couldn,t see though the glass that they were blocked ,but when i took the tops off they were choker,if the overheat was very quick it could have been a blocked raw water pickup which dropped off when the suction preassure decreased when engine on tickover?,
if you need an expert ,Doug Ashley is your man tel 01493 488000 or 07818040711,before you buy 500 pounds worth of parts,he may be able to pinpoint the problem for a couple of hours work,he,s very approachable and has worked on mercruiser since he was a boy.
hope you get soeted soon
trev