GE Dishwasher GDF510PSD1SS - lower arm skips spinning

[sbi123]

Greetings,

I noticed that the lower arm in my DW only spins every other cycle. I will try to explain.

Washer cycle on, I can hear the arm spinning and when I open the door I can see the arm slows down.
Then, what sounds like a "reverse" action, the cycle stops, I can hear what sounds like a relay/a few clicks, then another cycle starts but this time it doesn't spin. 
That cycle then stops, another relay clicks, another cycle starts and it spins again.

Does anyone have any idea what could cause it?

Thanks.