I've got a starter issue, could use some input

[Poky71c10]

So i've shimmed my starter to my flywheel with the appropriate 1/8" gap. I tighten it down and drive for maybe 200 miles and the starter starts to get the high pitch grinding noise. So i add more shims and then it engages perfect and sounds great. I drive another 200 miles and the high pitch grinding starts again so I add more shims. I've done this process 3 times. Any ideas?