[Steve Tinker]

The fluctuating temperature is a good sign that you have air pockets in the cooling sytem. I would not drive the car in this situation.

1) Replace coolant cap and check for a cracked resevoir, sounds like its failing to hold pressure (the latest cap version has a suffix 04 if I remember correctly.
2) It sounds as though you topped off the system with with coolant from a gas station - be very carefull as the Porsche coolant does not mix with a lot of commercial type glycols and causes the coolant fluids to gell. Not a good proposition.
3) Once you have sorted out the resevoir & coolant problems, open up the bleed valve (mounted on top of the coolant resevoir) for at least 3 or 4 heat / cool cycles to get rid of the air pockets. Check coolant level after each cooldown and top off as required.

If you are not confidant with the above, get a competant Porsche shop to replace the resevoir /coolant and bleed the system. We have a very good shop owner on the Forum (JFPinPA) who may be able to help.....