Also - kindly post back if you're already in an app using HDMI, or at the desktop when that freeze happens.
Also - just to round out the info - are you on auto resolution?
Finally - if one of you with this problem wants to guinea pig if this will work - the ever-popular -
http://androidforums.com/evo-4g-support-troubleshooting/141369-how-fix-froyo.html
Honestly - I can't see that helping - but - there is a software component to auto-sensing, so... ya know.
And finally - I know they don't grow on trees - but maybe helpful to have those with and without working HDMI on this issue to post their cable lengths.
So far as how it SHOULD work - that shouldn't matter one iota where we're talking about just plugging one end in only - no length-specific timing reflections regarding handshaking SHOULD be at play.
I'm just putting out a call for all data so we can track this bad boy down and put an end to the mystery.