i found out due reading a lot of forums posts on gtaforums.com that Realtek Drivers are the Problem (nforce 4 ac97 on board). I installed latest version hoping that this will fix it. BUT: it was worse. Game did not crash after 15min. it crashed after 1min. So I took my mainboard driver disk and installed A3.66 Version of Realtek Drivers (removed the newer one before).
Worked like it did after I bought my new pc. So Updating drivers is not the best thing to do everytime
This cost me about 5hours of play time.