FPS problem

[TheWeirdness666]

My cousin is running Windows XP Professional SP3, with 702mb of RAM, a AMD Anthalon XP3200+ processor, and has a Nvidia GeForce FX5500 graphics card. He downloaded AssaultCube, and he starts it up, and the first few seconds that it is running, it is at 50+ fps, but after a few seconds, it drops down to 2 fps. His drivers for his graphics card is up-to-update, and there isn't any problem with any other games that he plays. He has turned the graphics settings in AC all the way down, and it still does the same thing, what could the problem be?

Thanks in advance for reading and replying
~Will