VAIO weird gaming problem - please help!

i have a VAIO VGN-FS415M with a NVIDIA GeForce Go 6400, running windows xp. whenever i play games, mostly egoshooters, for the first few minutes of a map there's lots of freezing - sort of a bad fps but actually like a random pattern of on average a second or two of freeze followed by a second of normal, decent fps. this gradually gets better over a few minutes, and then the game is smooth and completely playable. it seems like the map loads before the computer is actually ready to handle it. this happens on both single and multiplayer games/game modes. examples include battlefield 2, halo/halo 2, and enemy territory - which is old and simple enough that there really shouldn't be any video problems. it seems strange to me - obviously the system can handle the game if it does work smoothly after the initial shakiness, so why the schizophrenia? of course i've searched all over already for someone else who's had this problem but to no avail, partially because the description of the problem is a bit messy for a search. anyway please help if you have any ideas, it's pretty annoying, especially when playing online - sometimes i have to sit out half of each map!