What version of firmware are you on? Can you try updating to the latest firmware 2.8.1. Instructions are in the link below.
Can you also replace the sensor batteries and let us know if this fixes the tamper issue?
If it does not, then it sounds like it is the sensor tamper button that is having issues and you would need to check the sensor and make sure it is closed correctly. If it is, then it may be the sensor that is having hardware failure and would most likely need replacement.