Strange Problems/Any way to completely clear out an eTrex Vista?

Thursday afternoon, my visa quit getting a lock. I did the click
up/page/power reset, which cleared out my user data (but left the
mapsource maps). The Sat. page said there were at least 12 sats above,
and it had black bars on at least 6, but no lock. I tried to tell it I
was in a new location and to search, same thing, although the message
at the top went from track satelites to searching for satelites. Again,
the same thing, no solution but several strong signals. I don't think
it was local interference, as I drove about 100 miles checking all the
way, and tried it about 150' from my truck that evening.

The really strange part is that it was working just fine when I went to
work in the morning. I do a lot of driving with short stops, and I
noticed it lost position (need clear view of the sky) right before all
this started (again, with 6-7 satelites locked).

Yesterday I left it at home, figuring something got screwed up in the
firmware, maybe. This morning I fired it up to reload the firmware, and
it got a solution, right away and with only 4 satelites.

I figured I'd go ahead and reload the firmware anyway, just for the
heck of it. After the restart, I was suprised to see the Mapsource maps
still loaded, and the unit didn't need to search for satelites, as I
would have expected. To reiterate, I was reloading the current
firmware, not upgrading.

So, to make a long question short: Is there a way to completly clear
out everything on an e-trex unit (specifically the vista)? I'm talking
making it look like it just came out of the box.

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