Had an odd thing happen yesterday.  I was flying from KMDW to KDTW via this route: LEWKE GIJ LFD MIZAR3.  Everything was going great using LNAV and the MCP for vertical guidance. After passing MIZAR intersection and half way to RAZIM intersection, the FMC popped up the message "waypoint not found", and the 737 immediately entered a right turn.  I turned off LNAV, and got back on the beloved magenta line via the MCP and HDG control.  Once I was back on course, I changed the FMC flight plan by mov