I had the same problem a few times but it magically fixed itself. Does this happen every time you drive the 'ring at night? Have you tried to see if it happens in arcade mode?
You have the whole 8gb or so installed in your HDD for faster loads? Is the game updated to its latest version? Has there been any recent problems with your PS3?
This happened to me aswell the first couple of times i ran nurburgring at night but havn't seen it since and raced on it alot, have a 40gb fatty fully updated and all installed.
Not much help i guess, but maybe it will fix itself for you too?
I had this happen once in an online race. It was daytime, and parts of the Nordschliefe and the surroundings of the GP track were missing, but for me they came back in a lap. I haven't done the installation.