When to let go?

[Perfectlap]

ALL convertibles rattle by the way, if anything this is the convertible that rattles the least in my experience. You simply can't hack the roof off a car and not expect things to shake when going over bumpy roads. ALso, are you checking your tire pressure regularly? This something you can get away with not doing often in a sedan but a convertible will let you know right away, same goes for your springs.

p.s.
If repairing your Boxster is starting to bug you that's sorta of a way of telling you that you'd like something newer. I know guys who think nothing of spending 3,000 on their 993's and would never consider updating to a newer Porsche.
The Carreras and Boxster of the last ten years were mass produced and the supply is abundant maybe higher than demmand right now, might be a good idea to get some cash for your car while the credit markets are starving for car and house buyers. That may come to and sooner than later.