First of all, not all TVs are capable of 720p. Check the manual for your TV to see if it supports 720p at all.
Second, it has been reported that certain TV sets had compatibility issues with VOOM STB at 720p over DVI.
Sorry to disappoint you tallguy, but Mitsubishi WS-55311 does not support 720p! Here is what the manual says:
Compatible DTV signals are SDTV 480i, 480p, and HDTV 1080i. All other DTV signals, such as 720p, need to be converted by the DTV receiver to one of the compatible signal types. Please check the DTV receiver specifications before connecting.
So, you should use 1080i fixed mode with your TV set.