Many times the problem is the refresh rate and or resolution. 720p is 1280x720, I would try 59hz and 60hz at that res. Also, some 720p tvs actually are 1366x768 resolution, try that with 59 or 60hz.
The easiest way is to have 2 monitors temporarily hooked up to the computer, 1 to control the settings on the vga monitor and the other to the tv. That way you can make repeated changes to see if it works.
Your video card may need to use an option like "Force HDTV" or Force 720p or something like that.