Cycoactive has a page section devoted to this problem. While they don't show what's done to "harden" the unit internally, they do imply strongly that the temporary "freeze" problem is associated with power supply. Since you said yours is hard-wired, the only suggestion I could make is that the wiring isn't sufficiently robust for the vibes your machine is getting when you're off-pavement and it's losing power. Take a look at the Cycoactive page here: http://www.cycoactive.com/gps/gps_vmod.html
Great timing on this thread. I literally just had my GPS "hardened" by Cycoactive here in Seattle on Monday of this week. According to them, the work they do is sanctioned by Garmin and does not void your warranty. After they do whatever it is they do to harden your GPS, they put a Vmod sticker oh the back that lets Garmin know they've done the work (just in case). According to the guy I spoke with @ Cycoactive, some thumpers set up a perfect vibration that causes the above symptoms - less so with BMW boxers. However, off-roading pounding can also cause the unit to freeze up.
I had the Vmod done because I'm offroad a bit and have pounded on the bike from time-to-time. That, plus the fact that I plan on keeping my 176c a long time. At $40 for units not purchased there...it's an OK price. JMHO.