Fluctuating hd free space

Question

I'm getting scratch disk full messages because my start up disk HD is wildly fluctuating the amount of free space it is reporting.
It is going from 168GB free (+50%) to zero KB and anywhere in between every few seconds. Even now while I am typing with only Mail and Safari running. I have 10GB of RAM so the scratch disk shouldn't really need to use the HD.
I've tried Disk Utility from Install Disk which reports nothing wrong, AHT - no problems, Onyx maintenance and cleaning.
Console is repoting:
Nov 29 12:42:33 CK-MacPro KernelEventAgent[35]: tid 00000000 received VQ_LOWDISK event (4)
Nov 29 12:42:33 CK-MacPro KernelEventAgent[35]: tid 00000000 type 'hfs', mounted on '/', from '/dev/disk0s2', low disk
Nov 29 12:42:33 CK-MacPro KernelEventAgent[35]: tid 00000000 found 1 filesystem(s) with problem(s)
which I presume is when the HD space is going down to zero.
Also - (not sure if this is related):
Nov 29 12:31:20 CK-MacPro mDNSResponder[16]: CK-MacPro._afpovertcp._tcp.xxx\.xxx.members.mac.com.: bad signature
Nov 29 12:31:20 CK-MacPro mDNSResponder[16]: Error -65560 for registration of service CK-MacPro._afpovertcp._tcp.xxx\.xxx.members.mac.com.
Nov 29 12:31:22 CK-MacPro mDNSResponder[16]: _autotunnel._udp.CK-MacPro.xxx\.xxx.members.mac.com.: bad signature
Nov 29 12:31:22 CK-MacPro mDNSResponder[16]: hndlRecordUpdateReply: Registration of record _autotunnel._udp.CK-MacPro.xxx\.xxx.members.mac.com. type 33 failed with error -65560
Nov 29 12:31:22 CK-MacPro mDNSResponder[16]: CK-MacPro._device-info._tcp.xxx\.xxx.members.mac.com.: bad signature
Nov 29 12:31:22 CK-MacPro mDNSResponder[16]: hndlRecordUpdateReply: Registration of record CK-MacPro._device-info._tcp.xxx\.xxx.members.mac.com. type 16 failed with error -65560
Nov 29 12:31:24 CK-MacPro mDNSResponder[16]: _kerberos.CK-MacPro.xxx\.xxx.members.mac.com.: bad signature
Nov 29 12:31:24 CK-MacPro mDNSResponder[16]: hndlRecordUpdateReply: Registration of record _kerberos.CK-MacPro.xxx\.xxx.members.mac.com. type 16 failed with error -65560
Any suggestions?

Answer 1

Can I completely trash Mail and all its prefs and reinstall it?
That may not be as straight-forward as it first it appears. 
First if you did that you would have to reestablish all your settings (accounts, rules).
Second some of the mail-related stuff is kept in non-obvious and not readable data bases and I don't know the implications of just deleting the "obvious" stuff like ~/Library/Mail and com.apple.mail.plist.  I'm thinking here of references to ~/Library/Application Support/SyncServices info.
Here's one thing that a "little" safer that you might try but even with this make sure you have backups.  In ~/Library/Mail you will (I think, based on what I see) see a folder with the name POP-f(account), where f(account) is some naming convention which is a function of your account.  Inside there you will see the various mailboxes folders -- names ending with .mbox.  Delete the appropriate mbox folder.
Deleting the mbox I think deletes the actual data.  So hopefully you can launch Mail and not get hung up on the large amount of data represented by those message.  However, those data bases I mentioned earlier will still think the data is there and show the messages in Mail's display.  So  you still need to try to delete those messages in Mail to get the data base back in sync with the fact that those messages are not there.  No guarantees this is going to work and have that backup in case it fails.