gpu usage problem

Hello everyone! Recently I had the opportunity to put together a 6x 1070gtx mixed brands.
When I use hashcat piping one of the c++ scripts found on here ( https://hashcat.net/forum/thread-6170.html ) the GPU usage is not 100% all the time. I'll explain better with pics.

HW:
  -6x 1070gtx
  -CPU i7-4790
  -MOBO H81 Pro BTC R2.0
  -RAM 2x 8GB 1333 MHz
[/url][url=https://www.ebay.co.uk/itm/Intel-Core-i7-4790-4-cores-8-threads-LGA1150-Haswell-tray-OEM-Processor-CPU/173553657891?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649]

SW:
  -Ubuntu server 16
  -nVidia driver 384.90
  -hashcat 4.2.1

If I run:
./ua | hashcat64.bin -m2500 -w4 lollo.hccapx
I've notice that the GPUs go from 100% to 0% and vice versa all the time as the pics shows. I guess that is the CPU not able to create enough

  hc1.jpeg (Size: 479.86 KB / Downloads: 0)

  hc2.jpeg (Size: 459.36 KB / Downloads: 0)