If you have not already done so I would go for a clean install of the GPU drivers and if this does not solve the problem go to http://www.trendmicro.co.uk/products/free-tools-and-services/index.html and download "Hijack This".This tool will produce a report of apps and services currently running on your rig and with a bit of googling you can usually figure whats running and why.Another possibility is the card starting to fail or perhaps needs reseating on the motherboard.
Update: It was indeed a bitcoin miner I had somehow been 'infected' with. Removed it, and the associated service.Have been closely monitoring the system since then, but it hasn't returned. Card is working fine again, and games are running ok again too.Be careful out there!Regards, Patrick.