MSI R9 290X Lightning problems:

Hey all, I have had a little stress recently after finally convincing myself to upgrade my GPU - I get a 290X lightning all set up in my case and I've had no end of trouble with random freeze/stalls or instant reboots. So assuming the the GPU was faulty, I went ahead and sent for a replacement (Bought from Amazon).

Convinced that if this second GPU was causing problems, that it couldn't also be faulty - there must be something wrong with my system set up. Sadly, I had the same problems. (Instant reboots / freezing / stalls) 

With both GPU's I managed to at least pass a benchmark or two without stalling. I even played Thief for a while on the second one. It didn't seem to have any issue doing what it was made to, but would often stall/reboot on simple desktop tasks or VLC.

I did assume that maybe it was a software/driver problem and actually re-installed Windows and installed just the 290X Catalyst Drivers but to no avail, it had the same problems. 

The only other thing I can think of: The PSU is not strong enough. But I struggle to believe this... and have sent the card back for a refund. NOTE: Both boxes look like they had been opened prior to my delivery and thus I will not be ordering from Amazon again for these high cost purchases, nightmare Amazon Logistics delivery services IMHO.

Sorry for the block of text but any help would be appreciated! (I really want to buy a new 290X!)

My Current System:

CPU - AMD  Phenom II X4 965

MOBO - ASUS M4A78LT-M

RAM - 2x4GB Kingston Hyper-X Blu

GPU - Sapphire HD 6850 1GB (Before MSI 290X Lightning(s))  

SSD - SanDisk 128GB

HDD1/2 - 2TB WD Green / 320GB Samsung 

Monitor(s) - 29" LG 29UB65 & 23" LG (Forgotten Model) Monitor 

Case - Corsair 750D

PSU - Cooler Master Silent Pro M, 700 Watt, 80+ (Bronze) - NOTE: Yes the CM PSU does have two separate 8 pin PCI connectors (Each cable has 6+2 and 6 pin connectors) 

EDIT: 

Operating System:  Windows 8.1 with Update 1 Pro N 64 Bit

For some further information for those wondering:

Yes I tried using all connectors (6+8+8 Pins) and just the two required (8+8 Pins) and both yielded the same problem.

I also tried the LN2 Bios which usually only made things worse (an actual blue screen sad face crash (hehe) in Windows 8)