I felt compelled to write this after just now fixing a week of frustration and finding others with the same frustration. There a many similar posts without an answer so I hope you all read this.
The short answer to all those who have 'legacy' Radeon GPU is to do a clean install of Windows 10 build 10532 (or I guess newer as this post ages). This was my very last choice but I ended up doing it because other drivers were also failing and the months of technical previews must have dirtied the system.
The long story might help others understand what I tried so what to skip. I spent many days troubleshooting BSOD from video_tdr_failure. Then on Aug 9 I thought I had fixed it ... https://community.amd.com/docs/DOC-1313#comment-4295.
But no. Within a few hours tdr_failures occurred again repeatedly. So I tried what others wrote about ... creating TdrDelay = 20 and TdrLevel = 0 registry entries. All that did was prevent Windows from automatically rebooting. Only to fail again and reboot. In an cycle that ended with windows giving up with Advanced Troubleshooting screen.
I was determined not to settle for AMD's 7/1/2015 driver's 1280x1024 highest option on my wonderful 27" 1920x1080, i7, SSD, Radeon HD 4800 machine.
So I thought perhaps the 1920x1080 was too much for the cobbled together windows tech preview and slew of drivers. So I tried 1776x1000 but still tdr failure. Then dropped to 1680x1050 but still tdr failure. then 1440x900 and some stability.
I turned off Cleartype Text at the higher resolutions but still tdr failure.
Finally when I couldn't take the 1440 resolution and knowing that the tech preview builds had junked up the OS and knowing that my audio driver was also clobbered. I decided to take the plunge by clean installing windows 10 latest build.
I'm glad I did. It's been stable all day. No video_tdr_failure so far. 1920x1080 resolution. Windows is using AMD's Jan 2015 driver. That driver allows HDMI sound through my monitor. I also got my native external speaker audio to work again. So many things were fixed with this clean install.
Hope this helps others.