So I've done a fair bit of research on this over the last week or so, and I've pretty much confirmed that the card is overheating and dropping the core's clock rate to cool itself down, and immediately ramping up the fans again. The issue sounds to be a quality control problem with the cooler mounting for the GPU. I've had the card for two years now and it's out of warranty, though I'll try contacting XFX anyway.
I've tried tightening the screws that hold the cooler on and I've changed the layout of my case slightly to ensure better airflow (took out a redundant HDD tray, redistributed some of the cabling so it's out of the way), but neither has made any difference.
I'm not *too* bothered about using framerate limiters on games that support them, or I can underclock the card, but it does effectively mean I don't meet the minimum spec for VR as I should. My only solution to that is either to take the card apart and either fix and reattach the existing cooler or buy a third party replacement, or simply upgrade the card once the new generation models are out.