I have an 86 Mariner 150 that is showing it is running very warm to hot at higher speeds. It has a new water pump kit, rebuilt poppet valve, 2 new 143 thermostats (tested good) new temp sender and seems to have no problem moving water out the tell tale hole. By the time I idle from the ramp to end of the no wake zone (approx 150 yards) the temp is showing at almost half and rises to 3/4 on any acceleration headed to H unless I back off the throttle. The water temp today was 90 as this is a power plant lake and is generally warm. At the advice of a local Merc service center i have removed the thermostats and put in blanks in their place just to test, made no difference except that I get a tell tale stream immediately instead of having to wait for them to open. I tested the water temp coming out of the tell tale with a laser thermometer with the gauge at a little over 3/4 and the water was at 148. The stream is constant, no sputtering or any signs of issues there. Im starting to wonder if this isn't a gauge issue. My alarm horn seems to have fallen apart at some point in the past so I cannot rely on that until I get it replaced. Has anyone ever had a gauge read incorrectly or is there something I am overlooking?