Where did your ET come from?
Here's a theory - Plot your Platelets from diagnosis to the peak.
Add a linear Trend Line, then extend it backwards in time to the point where it hits the min spec for Platelets.
Then mark the point where it crosses the max spec. Highlight the area, and this should tell you what period in time the condition started?
I've tried it, using the spec of 150-450, and come up with a period roughly between June 1992 and October 1994. See my chart
Now, did anything of significance happen during this time?
Flu? - Xmas 1995
Operation?