Looking quickly at the switch matrix, it doesn't look like the ball locks are on the same strobe/return as any of the tilts. However, it probably still relates to the switch matrix. I am new to Williams pins ( mainly a Bally guy ) but I have had this type of mystery problem several times and it was always one of 3 things : 1) Somewhere under the playfield there is a ground short. Go to the source of the problem first ( ball lock switches) to see if a switch ground is touching something that it should not. 2) switch matrix connectors. On Bally, the pins of the connectors were often problematic. Probably not the case with Williams, but the connections to the mpu might still be the problem. inspect IJ08 and IJ10 on the mpu if you haven't already. 3) Dried out diodes. However, if the diodes were the problem, the tilting would be more random than you have described. If you really wanted to remove the diodes from the equation, you could cut them as well.

Hope I gave you something helpful.