Issues with custom waypoints

I was flying a route from PHNL to PANC that required lat/long waypoints be entered in the fmc this morning and noticed that with LNAV engaged upon passing the custom waypoints, my -800 would turn about 45 degrees to the left of the magenta line and would continue heading in that direction. I disabled LNAV and used heading mode to steer back on to course but the aircraft would make the same 45 degree turn as soon as I turned LNAV back on again. The only way to stop this turning and allow LNAV to track the proper course is to activate direct to for each of the custom waypoints. Any ideas?