I found this picture from Mustang Monthly article showing a 65/66 switch with the spring. Since 67 is similar, I thought it may be the same.
I've swapped solenoids and the intermittent problem persists. I can usually make it work by jiggling the key in the crank position. I had recently removed the lock cylinder and thought maybe I,d lost the spring.
I also removed the cylinder from another 67 and found no spring there - maybe this is just a 65/66 thing.
I guess next step is to try a new ign switch unless anyone has a better idea. Hate to change out the orig. date coded switch.
Kurt.