I would go over the hoistway switches, magnets, induction switches, Interlocks and gate switch too. Do you have the prints? Do you know if it stops shy of the floor on an up call or is it stopping once it leaves the floor on a down call? Sometimes the induction switches can get stuck open or closed depending NC or NO. Prints should show, and test with meter. Are you resetting the elevator to get it back in service? I would wait until the next failure and dig through the prints especially the safety string. I typically bring our relay tester to jobs like this, that is if you have one. It can save quite a bit of time and frustration.