Pixhawk USB disconnects after ~30 seconds

I have gotten my Pixhawk and am attempting to configure it for my first copter.

The issue:
When I connect the pixhawk to apm_planner or mission planner the startup sound plays, and I am able to connect to the pixhawk for around 30 seconds before the link dies. This has happened on mission planner, apmplanner2, and qgroundcontrol on my windows 7 x64 system as well as apm_planner2 on my linux systems. After connecting to the pixhawk I am able to see the initial setup options and real time mag data through the GUI, then a preArm: RC not calibrated warning shows up and the board disconnects.

The Configuration:
I have a ezUHF rx configured for ppm and connected to RC IN on the pixhawk, as well as ESC’s 1 - 4 connected. the 3DR GPS/MAG, buzzer, and safety switch are connected as well. This happens with either usb or BEC powering the pixhawk. No logs show up on the pixhawk SD card, the firmware version is

I am having this same issue. Sometimes the link works when I reboot everything, often it does not. I have had this on both Windows and Mac. The problem has been getting worse.

The USB Connector on your laptop is probably not supplying enough current to operate the Pixhawk properly. You may need to use a powered hub.

Hi guys,
try changing the supplied usb cable - I found the supplied cable caused no end of issues. (Perhaps I was unlucky ?.)
However, many of the issues reported - such as difficulty installing firmware - no response from board - and also intermittent working - I had them all ! and they were all down to the cable. - sometimes it would appear to work then lose connection. It is now in the bin.
I tested the cable also on 2 other makes of flight controllers - both failed.
A good usb cable sorted all my issues - perhaps it will help yours ?.