I've had an 8790 for 3 years and recently had to reload my OS due to a HD going bad, and I was reminded about all the video problems all over again.
I have always had the video instability issue on my 8790 from day one and there is really only one fix. Determine the video core GPU and video memory settings that are stable for your system, and use them. This clocking "sweet spot" seems to vary from 8790 to 8790. The default clock settings are 410/270 (GPU,MEM), but this is way off IMHO.
For my 8790 I have to underclock the video memory speed from the default value of 270 down to 200 or my system is unstable under really any video loads. If I didn't underclock, I wouldn't be able to use my system and I would be very unhappy!
Also after extensive testing, I found that my GPU clock can be increased from a default of 410 to 525. So even after underclocking the memory, then including overclocking the GPU to this sweet spot, I get an overall frame increase over the default GPU/MEM settings of 15-20%; all because I had an unstable system originally due to bad default clock settings.
I have been running this 525/200 setting for almost 2 years and it is the only way that I would be able to run any games on my 8790. I know that it may seem daunting to overclock or underclock the video card (or both!), but the Omega Catalyst drivers make it very easy, and I believe it is the only real fix without replacing the motherboard.
If you aren't comfortable overclocking to look for more performance, at least feel free to underclock to increase stability and to reduce artifacting. This is a safe and easy thing to do, and beats replacing the mother board. You may not even have to go as low as I did, I think a lot of other 8790s are running at 240 or something. Like I said, it varies and you need to test it out on your system.
Here is a very good rule of thumb that helps when changing video clocking settings.
If the system is unstable and prone to lockups, this means the video memory is getting too hot, and needs to be underclocked a little. If the system artifacts or has weird video behavior, this is the core GPU getting too hot, and needs to be underclocked a little.
If you need to underclock both the GPU and the memory to very, very low levels just to run, they you have more than clocking problems. You have overall heat problems. You may have a blocked fan grille or rear air port. You may have a heat sink connection issue, in which case you may need to re-apply thermal paste and reseat the heat sinks on the vidoe chips. I've seen threads on how to do this, but I'd save that for a last resort.
Finally, and I know this has been mentioned before, clean out all the air vents, under the keyboard, and on all the grilles, and clean them a few times a year. I always find quite a few dust balls building when I clean mine out. I also elevate the back of the laptop to help airflow in general, and always use the fans on F10 high under video load.
My tips on 8790s.