I found exactly the same sort of thing when I had progressive fork springs. Mine were Yacugar branded but they are made by Hyperpro. After much experimentation with oil, preload, fork position and sag I ended up ditching them for Ohlins linear springs which cured the issue.

My trials and tribulations are all explained in this thread:

http://www.xt660.com/showthread.php?t=19823

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