Something isn't falling into place for me. Cure firmware simply prevents the loading of files essential to the proper normal operation of the device (i.e. map, voice, POI etc. files) and therefore the full boot process isn't completed. I cannot think of any other common software problem capable of causing the PC connection to be lost, that's a symptom universally due to corrupt file/s. Some thoughts:
  • As you summarized in your initial post, a hardware fault doesn't seem at all likely because it can access Mass Storage Mode with cure fw loaded. If hardware was the problem there'd be no MSM access at all. Access only with cure fw indicates a software or file problem generally.
  • After formatting there are no files in it, other than following a complete boot cycle the device self-creates GarminDevice.xml file in the Garmin root folder.
  • Your problem re-occurs regardless of which original firmware version is loaded so it's unlikely to be related to corrupt or buggy fw.

The only possible conclusion i can make is that the ability to access MSM disappears as a result of the boot cycle completing but not due to a problem with the original (non-cure) fw itself, rather due to some fault in the device loading the firmware.

Some more questions:
Have you recently tried to manipulate the firmware e.g. by attempting a cross-flash with a different device's fw?
Do you have a copy of the device's nonvol memory made before the connection problem started?