well scheduled scan rarely starts on time that could be the issue . Does it usually san on time and does it scan when the desktop is hiberinated?
I can give u a fix for the starting on time issue. The scanning when waking could be caused by the scan not starting
there is a boot scan option but the default is minimal scanning.
Is the 78% showing stuck on same file? You can exclude certain large zipped and avi files from scheduled scan settings.