We've been using a V-LCD70P-HDMI monitor for a while, and for some reason, it has recently stopped working with any 5D it is attached to. When connected to a 7D, it works fine. We've only tried HDMI, as those are the the only cables we have.
Does the 5D work when it is not in record vs when in record? It does change the signal from HD to SD in record where the 7D doesn't. But if the 5D is not working either time you may have a bad HDMI port in the 5D, Try the 5D to something else like a HDTV.