peak to peak and sync voltage
should be 0.7 on the video and 0.3 on the sync pulses. Some dvr's dont like it to stray to far (assuming its analogue)
the fact it works on the same phase id say your vid voltages are ok.
Also a lot of dvr's arnt earthed (dc) but if you had your monitor connected and that was earthed drpiing the dvr earth wouldnt of proved anything.I still reakon its an ground loop.