Vibration in steering only on Warm Days

[Steve Tinker]

Boxsters are very sensitive to wheel balance - mine used to vibrate when the tyre was cold. I would suspect a faulty tyre or wheel - you say they have been checked, but an out of round tyre is hard to detect unless on a good balancer (Hunter?).
For a quick confirmation, replace the front wheel with your spare - at least that way you can isolate which wheel/tyre combo that is giving you the problem....