Apollo GX55 flaked out.

I went flying yesterday and my GX55 said "GPS raim alarm, position
error". It put me in some random seeming location about 50 miles from
where I was and had me going north. Eventually it said "no fix" on the
GPS page. I turned it on and off a few times but it never acquired. I
figured maybe the amplifier in the antenna had gone out, but when I got
home I got it to acquire and track while sitting still on the ground.
Thing is, after the initial fix it started indicating that I was going
north faster and faster until it settled between 700 and 900 kts...
so, it seems like something in the box itself has broken itself.

I'm looking for suggestions on what to do. The nearest avionics shop
is an hour drive away, so that's a pain to deal with. I can send it
straight to the factory, but they want $350 to "overhaul" it, whatever
that means. Is the avionics shop just going to send it to the factory
anyway, plus charge me $80/hour to remove it? Is there anything new in
the GPS world that is form/fit/pin compatible that I should consider
sticking in that space instead?

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