Overheating issue

[BoxsterSteve]

Could be the cap, the newest is 04; or it could merely be the car heat cycling and burping out air introduced while the coolant tank was replaced.
I had a bit of trapped air after doing my coolant tank, and just drove around for a week with the bail lifted on the top of the tank. No problems since.

[j.fro]

Hate to say it, but check the oil. Way too often, when overheated the M96 cracks a cylinder head and you get intermix. I really hope this is not your situation.
If it is, you can gets the heads done without getting into the bottom end. And, if you are getting heads repaired, get them ported.