[Steve Tinker]

This really sounds like air in the cooling system, but the 996 expansion bottle is different to the Boxster one, and I'm not sure if there is a bleed valve you can leave open for a couple of heat cycles to rid the system of air like you can on the Boxster.
Have a look at Rennlist.com (under 996 Forum) and use the search function - it may help with diagnosing the problem....
As a matter of interest, did your friend split the radiators (coolant and A/C) on both sides when he removed the bumper and removed the rubbish - mine looked reasonably clean untill I split the rads, then a load of leaves, grass, cig butts etc fell out and even more when I hosed out between the rad fins.