Pwm fan issues

I have 2 PWM fans set up on an arduino mega. Using pins 11 and 12 when I apply power to the fans they spin up to max rpm and stay. Once I get into the shakeit setup screen and click test they slow down for a second the go back to max rpm. After a few minutes they will cycle from full speed to off and back every 2-3 seconds. Not sure what I'm doing wrong...