xb-502 electric bike

I just bought three used X-treme xb-502 e-bikes in two weeks.
The gals I got them for like them very much, but...

One of them doesn't respond when she turns the handle - sometimes.

What would make the power stop?

It only happens if the bike is stopped. When it gets going it keeps going. Retrying the twist grip and turning the key off and on doesn't seem to help. The glitch hasn't lasted for more that 15 seconds, but still ....