Upgraded to GTX780 Ti - computer shuts down under load

Hi there, I have a 1 year old Alienware Aurora R4 and I decided to replace my 2 Radeon 7870's with a single GTX 780 Ti. I have successfully installed the new card and drivers etc and it seemed to be working fine in Windows 8.1. However, on loading up Prepar3d (FSX) and flying for 5-10 mins the whole computer just shuts down with no warning. It is fine when just in Windows and not in games. I have a 6 core i7 oc at 4.2GHz and monitoring the core temps they don't rise above 70. Monitoring the GPU temps is the same so I don't think it's overheating. I have an 875W PSU and I'm pretty sure this should be enough for the GTX considering it replaces two cards? The one thing I haven't yet done is upgrade the motherboard BIOS - currently on A07 and believe there is an A10 release - could this be the cause with the GTX being such a new card? I haven't updated the BIOS as I don't know how to and reading online there are some scary stories about bricking Alienwares - any advice? I also have 5HDD's installed - 3SSD's and 2HDD's - I wondered if these were drawing too much power from the PSU? Can always disconnect one if so. Performance before shutdown is fantastic so I'm keen to try and resolve this. I'm hoping it's a pretty simple job! Thanks James