I have the 500R t-stat's also and that has happened to me alot too. Where do you have the probe located on the tank??? i have mine on the outside but im thinking of moving it to the inside. I really wished i had read the threads about thermostats because i should have gotten the herpstat II it is alot more reliable. If it is possible switch to a more reliable thermostat, but if its not just keep watching the temp.