I am using the same kit.
I live 5 miles and 600 vertical feet from work.
On the way home, going uphill, with a fat 230lb man on
the bike the 500 Watt motor will not pull me home.
When the motor gets loaded near 500 Watts (around 14Amps)
the motor begins vibrating as the sensors request more current
than the controller (and the wiring) can supply.
This is normal, and means you are coming close to damaging
things. I bet your power LEDs dim too.
The supplied battery cable is a standard computer cord
rated at around 8 amps. If you check, the cables are getting
warm and the motor is starting to heat up.
This also sucks the juice out of your battery and converts it to heat.
When I ride, I listen to the motor and if it is 'buzzing' I back off the
throttle and pedal more. This gets me home and helps me lose some
of the extra weight I am carrying.
I am also planning on upgrading to the kW motor at 48V because
I want to carry groceries up that hill too.