my99jm

Q-See QR4074-418-3 DVR system I set the length of recorded files to 30 minutes. When I played back any 30 minute segment it worked just fine. Yesterday when I reviewed a couple of 30 minute segments I experienced that all segments ended 12 to 19 minutes from the start instead of the programmed 30 minutes. I removed the power, waited for 10 seconds then I re-powered the system and experienced the same thing. How can I fix this? Thank you in advance.