It's maybe a 'space' problem but i think having ~450MB spare is sufficient. I had Garmin units operate quite normally with much less than that. However, yes you can move the maps and associated files onto a microSD if you wish to try that.

About re-flashing the current V4.00 firmware as an RGN, again i don't see a problem trying that either although this just doesn't feel like a fw problem to me. More likely the symptoms you've described are due to either a partially-corrupt file onboard or, of a more problematic nature, a corrupted non-vol region, or, ultimately fatal, a failing flash chip. Like i said before though, usually that kind of flaky behaviour is fixed by clearing non-vol.

Certainly the other device discussed in the linked thread had some type of physical damage going on with its flash chip.

Try the other things first and if the slow functioning persists we can dump its non-vol region then erase it. After it boots and re-writes part of the region, if it functions normally we'll know that the problem surely lurks in the not-clearable (but erasable) data in that region. If its flakiness still persists then re-writing the dumped copy will be in order.

Unfortunately if the flash chip really is failing, any messing around with flashing and dumping might just hasten its eventual demise from the stress of it.