Alarm coming on mercury xr2

[bassmanrob]

I have a 1986 mercury xr2 it says a 150 but my local shop said it was rebuilt and now has a 200 block. My issue is I can be running wide open then the alarm comes on I shut it down wait a couple minutes then start back up run about 1/2 mile it alarms again. It has new water pump and impeller plus new thermostats. it did it before those were changed I'm stumped as to what it can be any suggestions?

[JOE54]

lots a stuff had to be changed to use 200 block on 150xr2, anyway sounds like its running hot , all 3 gaskets on pump , water psi ?????? check v6 section for more info on yo mtr