weird temp readings

[drummer0901]

82 blackmax 200 on an 18 ft procraft bass boat. Sometimes warning buzzer goes off at 140 sometimes not my temp guage gets to 180. I have been reading a bit on here and wondering if the sensor might be bad. I have no beep at start up. Over the last 3 years I have replaced thermos water pump and poppit. Temp tends to rise at partial throttle not on plane any help will be appreciated

 

[drummer0901]

Re: weird temp readings

Hate to double post but this one has me puzzled. I hear nothing out of warning buzzer until overheat. What is best way to test if buzzer is telling the truth. Water pressure seems normal