I am running a 1998 Mercury 175 EFI that I�ve been having problems with for awhile. On some trips the solid alarm will go off and other trips it won�t. On the last trip to LAH, I was making a run down the lake and the solid alarms sounds after just a few minutes on plane. I stopped and idle and the alarm stops. I bring it back on plane and the alarm sounds again. I stop for a minute and bring it back up and this time the alarm doesn�t sound. On the return trip back to the ramp the alarm did not sound the entire run. As I said, it is the solid alarm which indicates overheating. The thermostats were replaced three years ago and the impellor a couple of years ago. A good strong stream of water is coming out the motor. Why do I have a problem on some trips and not on others? Any help would be appreciated.

change the impeller if you dont see salt water the thermostats should be ok the impeller needs to be changed every other year if you dont use it alot but i change mine every year but i get alot of use out of it

and if there are any pieces of the impeller missing try to find them and get them out

I had a Yamaha do the same thing. Turned out to be a head gasket. Fought it for a year and had it to 2 different mechanics that couldn't figure it out. Finally took it to Rockwall Marine and they figured out the problem in short order. Replaced the head gasket and never had an issue again.

Could be the poppet valve.

Complete water pump kit first. A strong stream may not necessarily show a problem there. Have ya got a guage?
Changed motor height and/or runnin' at a different trim than normal when it happens?