Seeing as you have the non-adj type, the hole placement in your rudders are all you have to adjust the geometry. I suspect that the holes in your rudders are worn. You may have to fill and redrill your rudders. Your castings may be worn as well. I've seen where the hole that the plunger goes through widen and cause the plunger to rotate. This will also happen when the cam sister screw holes are elongated. The cam will move up and allow the plunger to slip through the space between the cam and the casting. Did that sentience even make sense? Bottom line is that something is worn out of tolerance. Try to grease up the area where the cam runs on the plunger and see what happens. That solves about 50% of all old rudder maladies. It's not the right way to do it, but when the real fix is time consuming or expensive, it's a good temporary solution to get back on the water. Try loosening the plunger adj as well. Send us some pics and we can better assist.
The adj casting will work in combination with your older upper.
This might help:
http://www.youtube.com/watch?v=zUUA7l6fzIk