External monitor displaying only noise after Yosemite upgrade
UPDATE

I almost forgot I had this issue, recently I changed cables. I am now using MDP to HDMI without adapters. Before I am using DVI to DVI Cable + DVI to MDP Adapter.

I think you need to avoid 2 conversions and use only 1 cable.

OLD ANSWER

It is also happening to me, and I can't find the solutions either. But I figured out how to avoid it. 

When booting up mac


turn off monitor first and then power on the mac.
turn on the monitor after the mac finished boot or in the login screen


Settings


Open system Preferences > Energy Saver
Click Power Adapter tab(assuming your mac is plugged in) and slide the 'Turn display off after' to at least 1 hr. 


It also occurs when mac goes to sleep and to avoid it, the steps above should be followed. Only do the settings step if you don't mind the power consumption. 

Please let me know if you have better suggestions but this works for me as of now.