MP 6,1 Mac Pro 6,1 Late 2013 D300's occasionally won't output video, sometimes get Kernel panic

[GreppMichaels]

I'm troubleshooting remotely for a client that I sold a 6,1 to, it has D300's and occasionally won't output video, it takes 2-3 boots sometimes for the GPU to output a signal however once its up and running there is zero issue. It needs to be rebooted several times, and also has shown this kernel panic:
virtual bool AMD7000Controller::detectPowerDown(): GPU is not found.

Is one of the D300's failing? They have tested other TB2 ports, and are in the process of trying different cables to test as well.
Thanks in advance for any ideas!

 

[StoneJack]

maybe you should try internal video for testing? (no idea. if the mac has internal video), if KP persists, maybe something else. If it is ok without card, time to check the card