Multiple WU's causes failure

I need some advice please because I find this issue interesting...

I've tested 2 different RX 480's running 1 WU which works fine. However with both cards, if I increase it to 2 x WU's, after say...1 min (maybe not even 1 min), the screen freezes for a second, goes black (device hanging?) then recovers but the GPU load drops to 0.

In this state I can run Furmark with no issues. I restarted GPU-Z just to check it wasn't falling over (unlikely) and it's fine. It still reads 0 GPU load with BOINC running.

If I quit BOINC and restart, 2 X WU's begin again then fails.
I'm left thinking that it surely can't be the cards and the PSU is a brand new 1050W 80 plus rated so I doubt it's the PSU.

Anyone got any suggestions?

I could pull my Vega out of my second box and test that. That would definitely exclude issues with the cards but that's a fair bit of work etc.

Thanks in advance.