Saphire Toxic R9 280X Cut out problem

Hi PC tech heads,

Here is a testing one for you as I am completely stumped.

First of all, below is the setup of the build I just completed.

Windows 7 Professional 64-bit
Intel Core i5 4670K @ 3.40GHz
8.00GB Dual-Channel DDR3 @ 665MHz
Phillips 274E5 Monitor (1920x1080@59Hz)
Saphire R9 280X Toxic 3G 5DDR5
1863GB Seagate ST2000DM 001-1CH164 SCSI Disk Device (SATA)
2 Intake fans on a 2 way splitter
2 exhaust fans on a splitter
1 intake fan
cpu cooler with x2 fans
Cooler Master GX 650W bronze power supply

My pc has been running fine using the built in graphics on the cpu, everything was running ok until I decided to get some thing more powerful. I have just installed a Saphire Toxic R9 280X, it has been running fine and I was impressed by its results.

The problem.

1) My monitor keeps going into sleep mode and says there is nothing detected, it happened a few times at first, now its permanent. I have plugged the HDMI back into the motherboard to use the onboard graphics instead.

Would this be something to do with the psu not being powerful enough, Saphire recommend a 750W power supply or could the gpu be faulty.... I'm totally lost.

All drivers are up to date.

Please help someone, I'm pulling my hair out here.

Many thanks

Scott