Weak laser. To confirm, eject the map and leave the map out for a while. Observe how the system responds. Most likely fast and stable with no nmap disc.

Then put the disc back in. If it is still slow reading the nav disc, and sometimes crashes, then your laser is old and weak.

Either replace the laser if you know how, or replace the whole mechanism (recommended). Replacing the entire mechanism is very simple: 4 x screws, 2 x plugs. That's it.

It's best to do the whole mech because if the laser is weak then likely the mechanics of the mechanism are worn as well. A whole new mech makes the nav as good as new.

New mechs at BMW or here.