I wouldn't know any .mp4 repair programs off hand, but this has inspired me to suggest to the devolopers to cut up Debut recordings. Similar to fraps.
Drop some options in the mix to change the when it automatically cuts your recording, file name prefixes and the core option to disable record splitting.
Ohjeez, this could work!
As for your problem; if google isn't helping you then the chances are that any solution probably doesn't want to be found. Or doesn't exsist.
EDIT: In the Debut window, up top, the 'Options' icon. Click.
Select the 'Record' tab.
Tick the 'Limit Maximum Recording Time' option. Set it to whatever, and select 'Create new recording and continue'
Found this about 6 minutes after posting. Iamsogood...