I am wondering if there is a loose wire within the mount and when the motors are running at full speed and drawing the most current that this becomes a problem.

 

With everything connected up it draws more current - though not 10 amps. The voltage stays on 13.8 volts but the low voltage light on the mount illuminates, wierd! I guess I just ignore it?

If you are using the top arrangement for your cable run as shown, it would help a lot to use separate cables for the supply to various high current loads instead of a single cable for all of them:

 

thats a good idea! will do that!

 

How long and what gauge are the wires from the PSU to your PWM unit? If more than a couple of inches, you really should have a large capacitor at the input of your PWM circuit.

 

The wires from the PSU are about 5m in length!

 

What gauge or mm are the conductors?

5 meters x 2 wires (power, ground) = 10 meters. Those are pretty long wires! You definitely need capacitors near your PWM circuit.

I would not go smaller than AWG 12 or 2mm wire if you want to provide for up to 20A current. 10 meters of AWG 12/2mm wire, if straight, would have about 18.3uH inductance, which could wreak havoc with your supplys' regulation. Add a 1,000uF 16v (or higher) electrolytic capacitor wired directly across the input of your PWM circuit should largely eliminate the problem you're experiencing.

[eta]
Straight wire inductance calculator: http://www.consultrsr.com/resources/eis/induct5.htm

 

Thanks for the info - will go buy one tomorrow