Seems video problem. Does it work with 1 or 2 gpu on the mb?
Try with only 1 gpu, then 2 , etc.

See if this helps

The Diagnostic LED blinks red 5 times and the system beeps 5 times (once per second), then a 2-second pause follows:
This indicates a pre-video memory error.
To resolve the issue, complete the following steps:
1) Ensure that the memory modules are loaded correctly.
2) Reseat the memory modules.
3) Replace the memory modules one at a time to find the faulty module.
4) Replace the third-party modules with HP memory.
5) Replace the system board.

The Diagnostic LED blinks red 6 times and the system beeps 6 times (once per second), then a 2-second pause follows:
This indicates a pre-video graphic card error.
To resolve the issue, complete the following steps:
1) Reseat the graphics card.
2) Replace the graphic card.
3) Replace the system board.