I am also seeing this lockup issue. MM will get a GPS fix, run for a while (5-30 secs) and the GPS seems to stop updating (MM still responds to some things like the close button). After this, sometimes I will see stray MM processes left around which I have to kill in order to establish connection to the GPS again (although sometimes even this does not work and it requires a reboot to get connection to the GPS again). Another data point that I have not seen mentioned is that after this happens NaviCore can no longer connect to the GPS either. Sometimes I can get the GPS back in NaviCore by going through the "Activate GPS" procedure but not always.

N800-OS2007 - 4.2007.26-8
MM 2.0.5
Nokia LD-3W as only bluetooth device

I'd just like to restate what many others have already said - this is an awesome piece of software and impresses the h*ll out of anyone I show it to.

Cheers,
Kris