I would start by removing and reseating the video card. Also reseat the MPU firmly. I had an intermittent problem with mine and that fixed it.
The instant the machine starts booting, the LCU is in diagnostic mode as it cycles each device through the colors Red, orange, yellow, green, blue, indigo, violet. Does your machine cycle through each color correctly?
I don't know if the LCU boards are interchangeable. I know that for the Alpha Pro Wave there are several different boards of different part numbers. I wouldn't try switching them.