HDMI does't always work on startup...

So most of the time the system comes up perfectly, but other times it fails to display a signal to the TV via HDMI. If I plug in a DVI cable it works perfectly. It's itermittent so I'm not sure what is causing it.

I could use the DVI but it's a pain to route that bigger plug, but if thats what I have to do so be it, but would rather use HDMI.

Any ideas what could be causing this?

System Manufacturer/Model Number Custom Sytem OS Windows 7 Ultimate 64 bit CPU Intel Dual Core 2 E5400 2.7 GHZ Motherboard ASUS P5KPL-AM EPU Memory 4GB PC5300 DDR2 667 mhz Graphics Card(s) HIS RADEON HD 5770 Sound Card AC 97 Direct 3D Sound Incorporated Monitor(s) Displays Sharp Aquos 42" HD LCD TV Screen Resolution N/A Hard Drives 500 GB Seagate 3Gb/s PSU Corsair TX750W Case Apevia X-Dreamer 3 Cooling 10 Fans - 3 Case/2 RAM/1 CPU/1 VC/1 PCI/1 HD/1 PSU

Does nothing appear on the screen at all if you wait while HDMI is plugged in? I have my main monitor on HDMI and my second monitor on DVI, and while the PC boots up the DVI monitor will have the display on, then when Windows has started, the display is on my HDMI monitor.

Nope, it just sits there blank, PC connected to surround sound so you hear the Windows log on audio but the screen is blank. Tried powering the TV switching inputs etc nothing, soon as I plug in the DVI, perfect! Like the signal is sometimes not going through HDMI for some reason...