It happens differently everytime. Two times i was on the runway ready to takeoff, and after switching to my takeoff page and flightplan on the other fmc, the fmc stops reacting awhile after not sure when exactly, one time everything was fine and when i was on my way to the runway the fmc freezes, last time the plane stopped following the flightplan and started looping down towards the ground. i was flying from pisa to basel,
Let me know if there is anything else that you need to know.