mozilla Thunderbird

Hi, I'm having a minor problem with mozilla thunderbird. when I first turn on my comp and open hunderbird, I have no problem getting or writing messages. after my comp has been on for a while, and I try to check my messages, i'll get the message " Unable to write to the mail box. make sure the file system allows you write privileges and you have enough disc space to copy to mailbox."
if I try to send a message i'll get the message " The message could not be moved or copied to folder 'sent' because writing to folder failed. To gain disc space , from the file menu the file menu first choose empty trash and then choose compact folders and try again"

tried what messages suggested but to no avail. However if I reboot, and use thunderbird, it will work fine again.

Problem doesn't occur all the time but sporadically.

My system is an AMD 64x2 4800 with 3 GB ram 320 gb HD and running XP
home edition. and hd has 93% free space. Any suggestions?? Thanks.