When it flicks between video sources (like opening a video and it changing resolutions etc) sometimes it goes completely purple, and everything else that opens is purple. The only way to fix this is rebooting the console, but it's happening more often now (like a few times a month).

Is it the GPU dying very slowly or something else?

I've got it running through HDMI on a Panasonic 40" Plasma, and it's happened to my other consoles in the past (which all died also).

Firstable check your HDMI cable.
And yes maybe the console is going to rrod...BTW what revision is your xbox?

Hdmi all good and ive got an elite, pretty sure its a jasper its a 2008 model that got fixed last year from rrod at ms

Edit: yup checked it was fixed on 22/12/2009

What's happening is the result of your GPU. This was happening to me on my elite (well it was alternating between all green or like your case sometimes purple) after I did the x-clamp mod. After the mod it was working fine for about 2 months then it started to happen. All I did was remove the heatsink from the gpu and cleaned and polished the heck out of the chip, then I re-applied some good quality heat paste (I use Dynex) and whaa-baam! been working fine ever since. If your box is already modded (the x-xlamps) then I suggest trying this. If you've never touched them before, then I suggest doing the clamp fix. In reality you probably just need to get rid of the old cracked and crusty low quality heat grease that microsoft uses and re-apply some nice shtuff to your GPU.

Sounds like an idea, but when it goes back to the dash, it's fine, not purple, it's just on videos?