Uninstalled with DDU, reinstalled drivers from back at the beginning of the year. Ran Furmark with Xtreme burn-in and Post-FX on 1080p for 10 minutes. Ran fine, only got to 81c. Jumped in a game, managed to play for 30 or so minutes with all graphics on max (lasted longer than it has before), but then it crashed again. I'm almost considering buying a new Mobo, CPU and ram just to rule out every bloody thing. Rather not have to if I can avoid it. Unfortunately, I don't have a spare mobo or cpu to test out, to see if they are the problem.
EDIT: Looking on the AMD forums this seems to be a relatively common problem, haven't found anyone with a solid fix though :(. https://community.amd.com/search.jspa?facet=content&q=RX480 crash&sort=relevanceDesc