If it's still under warranty, I'd phone Bell for a replacement. One of the problems I've heard with Bell HD PVRs is that the hard drive gets corrupted if it gets too full. Maybe something on this page will help.
http://www.digitalhome.ca/forum/forumdisplay.php?f=58
If your PVR is a 9241, the engineers are working on the problem and people should give the model of their equipment when asking about them.