how many mp3, videos and pictures do you have on your N900 and microSD? sometimes the pulse audio keep on scanning for any changes and freeze the device. sometimes he hildon crash and you may have to turn it off then take the battery out for 5minutes and start it back on. But your Virtual Keyboard might stopped working after the reboot.
Are you using the load-applet? It lagged my N900 badly when I used it... you could try to uninstall it.