It realy sound like your using an old version of gpsp. this is the latest version: http://notaz.gp2x.de/misc/gpsp09-2xb_2.zip
Also if your gp2x is going to crash after 15 minutes at 250mhz why not just over clock to 240mhz instead.... its not going to hurt. But its only 24/25 the performance zomg!