ATI Radeon HD 5770 (monitor) or Graphics Card?

I have been having a problem with 2 of my lab displays. They are several months old. The screens go black and the machine won't wake. The CPU remains on.

I switch the monitor port on my CPU and push it in. The monitor then works, most of the time. I may have to switch several time for it to work. The cable is not loose either (to begin with) I am extra careful with my connections to the CPU and ensure they are as tight as can be due to this issue. They are securely seated in the port and it is always on the same 2 machines.

I plan on switching the monitors on both CPU's with different monitors that I know have not had any issue to see if it is the graphics card or not

but thought someone may already be aware and have a solution to the issue.

I am running 10.6.8, Snow Leopard.

Model Name: Mac Pro

Model Identifier: MacPro5,1

Processor Name: Quad-Core Intel Xeon

Processor Speed: 2.8 GHz

Number Of Processors: 1

Total Number Of Cores: 4

L2 Cache (per core): 256 KB

L3 Cache: 8 MB

Memory: 8 GB

Monitor Info Below:

ATI Radeon HD 5770:

Chipset Model: ATI Radeon HD 5770

Type: GPU

Bus: PCIe

Slot: Slot-1

PCIe Lane Width: x16

VRAM (Total): 1024 MB

Vendor: ATI (0x1002)

Device ID: 0x68b8

Revision ID: 0x0000

ROM Revision: 113-C0160C-155

EFI Driver Version: 01.00.436

Displays:

LED Cinema Display:

Resolution: 2560 x 1440

Pixel Depth: 32-Bit Color (ARGB8888)

Main Display: Yes

Mirror: Off

Online: Yes

Rotation: Supported

Connection Type: DisplayPort

Display Connector:

Status: No Display Connected

Display Connector:

Status: No Display Connected

Has anyone else been having this problem?

Thanks