Chris,
Have you checked out your trunnion bracket for wear? I know that can cause lurching, but I assume it can also cause speed control issues.
From what I read, it's the nature of the hydro drive to want to run at full open, so it's all in the linkage to hold it from speeding up. Does your tractor tend to speed up?
<edit - ah, I missed the "tiller" aspect...>
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