my money says your chasing the wrong problem.
you stated that the key works fine in all the locks, and only gives you trouble in the ignition switch........
the ignition switch is a well known failure in these machines and i have had several of them apart and repaired them.
when they get overheated and get near failure, the switch contacts melt and drop down inside the plastic base of the switch and it gets hard to turn the key.
I would be glad to offer more details if you wish to hear them, and want to remove your switch for repair.
mike