have you tried updating your drivers? first video, then audio. Make sure you have the the most current versions of each.
here is an article you might want to read through and see if you can make any of the adjustments discussed to increase system peformance.
http://www.tweakguides.com/NFSMW_1.htmlAlso, since you say it let's you play for a while before kicking you out, you might want to check for actual system stability and overheating. Make sure that all your case fans, as well as your CPU fan, are clear from dust and debris, and that they are spinning correctly.