I've just notice this when re-reading post #1: ".... with car charger it still powers down.". That's not typical behaviour with cure firmware loaded. Maybe it isn't a physical hardware problem however that symptom isn't totally indicative of a soft-bricking merely due to a bad file. However perhaps it had an inappropriate formatting. Also, would you explain your mention of "(Ver 4.00)" in your first post, didn't you use V5.50 to make the cure fw?

I suggest you try another FAT32 format with RMPrepUSB, carefully ensuring the check boxes and radio button settings are as follows:



After successful format, add at least an empty Garmin folder or the files i supplied and then load original V5.50 fw to see if it performs properly.