There was an issue with rear subrame braking/cracking on earlier Boxsters. I beleive that for the model year 1998 or 1990 Porsche changed the rear subframe to a stronger one.
This is the subframe where some suspension arms connects to.
Take the car on the lift and inspect any cracks on the subframe. If it is cacked, it would most likely cause similar symptoms you described and in that case do not drive it untill you have swapped the subframe part.
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Boxster 2.7 2001 Manual
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