I have had the Oregon 300 for some time now ... Yes it can hang up on occassion.
I keep the GPX files on the 4 GB microSD card. By creating folders on the card that match those on the Oregon 300 internal memory ie. a "Garmin" and "Wherigo" folder with sub folders, you then send your GPX files to the card either through the unit or a card carrier.
I also "reset" or delete all waypoints on the Oregon 300 so I don't get Memory full.
Using this method, if the Oregon gets stuck or hangs up, remove the microSD card (safe with your GPX file) start the oregon, reset all waypoints, put card back in unit and restart.
This has restored my GPX file each time without fail.
I also keep all maps loaded on the microSD card.