Sounds like you might need to update the firmware. If you type *#0000# on your keypad it'll pop up a bunch of numbers.
The top line tells you your firmware version. The most current version as of typing this message is V5.0719.0.2 - if yours is lower than that, then you can update by downloading the nokia software updater. (NSU) from their website.
V5 is pretty stable, rarely crashes so badly that you need to reboot.