How much data have you got on the phone?
You could either try reducing the quantity of data you have if you have loads, or you could try backing up your phone, formatting it and then restoring the backup to see if that helps get it back to its usual self...
To format the communicator, remove and then replace the battery. Then when you see the word Nokia in blue on the inside screen, hold down the ctrl, shift and F keys simultaneously. After the word Nokia disappears and before the Nokia handshake logo appears, you should be given the option to completely format the device .
ALL DATA on the phone memory will be completely erased and the original settings restored so make sure you have any important data backed up.
If you didn't get the option to format the device before the Nokia handshake logo appears, repeat the same process again and hopefully you will receive the option before this logo appears.