1993 yamaha 200 alarm goes off

my motor takes off great but when trimmed out the alarm sounds. This happened before and I changed the voltage reg and disconect speedo it stopped. That was months ago but it started back two weeks ago. Installed new water pump ck termastats &popit vavle. I also de scaled water jackets even though freshwater only. Ck engine temp w/temp laser 115 when alarm going off. Oil levels full new filter in tank pump works perfect. Three lights on tach blink. I believe I have ground problems. I put new temp gauge w/bolt on temp sensor worked fine on hose. At the lake when alarm went off gauge went to 250 but never went down. When I touch the sensor to motor the gauge max out. Water pressure is 5@idle & 30@ 5000. Alarm sounds after just 2 min in 50? weather I know motors not hot. Thanks for help.