2 stroke 50 warning sound

Earlier today the boat was running great. Later in the day, once the boat was launched it started and idled fine. Once I got out of the no wake zone and got up on plane and the warning alarm came on with the motor running rough. It starts and idles fine with the alarm on, once it gets to a certain RPM it sputters. The warning sound is a solid beeeeep, not intermittent. Comes on as soon as the key is turned. Oil level is fine, the temp is fine. The guy I bought the boat from said he had just changed the thermostat. The impeller is pumping water fine. The motor is a 2000. Any help is greatly appreciated.

[boats.net]

The alarm will activate if overheats or low on oil and will reduce rpm's to 2000 rpm's. Could be something not hooked up correctly, bad sensor or wiring.