In one of your startup scripts put something like
but I don't know how advisable this is!! What is filling it up and not cleaning up after itself? If anything in you boot process uses /tmp as a tempory store, or to hold some data between boots its not doing to be very happy when you trash it!
If you post the distro you are using I'm sure someone will be able to give you some advise as to what file to put it in.