rental VTR’s get very beat up all the time. PLEASE simply put up bars and tone from your FCP system – output into your VTR, and put the VTR manually into an insert edit from the front control panel. It’s easy. You will tell in 2 seconds if the VTR has a problem.
I had a similar problem to you once, and it was due to an unstable black reference between the K-Box and a Digi Beta VTR. Make sure that both machines have a common black reference, and that the control tab is checked to Ext Ref, and not video in.
bob zelin