AMD Radeon R9 280X card in Mac Pro 2008 3,1 - Windows 7 via Bootcamp, problems.

Mac Pro 3,1 - 2.8Ghz quad. 14Gb RAM OS 10.9.5. We have Windows 7 running on its own HD under Bootcamp.

we installed an R9 280X graphics card already flashed for Mac OS when purchased. We also followed instructions and installed latest Windows drivers from the AMD site. My son has been using the Windows side for gaming and needed something with a bit more welly to play some of the games he downloads from Steam. However, there are various anomalies with some of the games. Notably GTA5 seems to run without issues but some games, even older ones which worked fine under the original nvidia graphics card, don't under this one.

Witcher 2 and 3, Middle Earth, Borderlands 2 and Rome II thus far all seem to cause the fans to spin up followed by the screen going black and the whole machine seeming to crash, which needs rebooting. The power light stays on on the front of the Mac though, just no signs of life until rebooted. Different games fail at different points, sometimes there is just some stuttering or screen jumping. We also conducted a bit of an experiment with Diablo III - installed it on both sides, Mac and Windows. On the Mac side it runs apparently smoothly, with little or no fan noise. Under Windows 7 the fans spin up but it doesn't crash the machine until a fair way into the game.

I have read elsewhere on the forum that this card may need an extra power unit - but am puzzled as to why it would appear to run without stress on the Mac side and yet not on the Windows side.

Can anyone shed some light on this, is it a conflict with the Windows drivers? Or if it needs a new power unit.. what sort?

With thanks.