Messages locks up on Yosemite

Question

My 2010 15" MBP won't run Messages. I *think* this started when my laptop crashed a couple of times, but I can't say for sure.
Messages had been working fine, but now when I click on it in the dock, the Messages window never appears, but it chews up a core (reported as 99.9%).
If I leave it running then I end up seeing the following messages in the console:
22/04/2015 15:46:18.164 WindowServer[176]: disable_update_timeout: UI updates were forcibly disabled by application "Messages" for over 1.00 seconds. Server has re-enabled them.
22/04/2015 15:46:32.165 WindowServer[176]: disable_update_likely_unbalanced: UI updates still disabled by application "Messages" after 15.00 seconds (server forcibly re-enabled them after 1.00 seconds). Likely an unbalanced disableUpdate call.
22/04/2015 15:47:45.000 kernel[0]: process Messages[11975] thread 573837 caught burning CPU! It used more than 50% CPU (Actual recent usage: 99%) over 180 seconds. thread lifetime cpu usage 90.055734 seconds, (87.894891 user, 2.160843 system) ledger info: balance: 90009103052 credit: 90013217585 debit: 4114533 limit: 90000000000 (50%) period: 180000000000 time since last refill (ns): 90804090504
I tried deleting all the files matching ~/Library/Preferences/com.apple.iChat.*, suspecting that preferences had got corrupted, but that didn't make any difference.
Any ideas what to try next?
Thanks,
Neil

Answer 1

Please read this whole message before doing anything.
This procedure is a test, not a solution. Don’t be disappointed when you find that nothing has changed after you complete it.
Step 1
The purpose of this step is to determine whether the problem is localized to your user account.
Enable guest logins* and log in as Guest. Don't use the Safari-only “Guest User” login created by “Find My Mac.”
While logged in as Guest, you won’t have access to any of your documents or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this behavior; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin.
Test while logged in as Guest. Same problem?
After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it.
*Note: If you’ve activated “Find My Mac” or FileVault, then you can’t enable the Guest account. The “Guest User” login created by “Find My Mac” is not the same. Create a new account in which to test, and delete it, including its home folder, after testing.
Step 2
The purpose of this step is to determine whether the problem is caused by third-party system modifications that load automatically at startup or login, by a peripheral device, by a font conflict, or by corruption of the file system or of certain system caches.
Please take this step regardless of the results of Step 1.
Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards, if applicable. Start up in safe mode and log in to the account with the problem. You must hold down the shift key twice: once when you turn on the computer, and again when you log in.
Note: If FileVault is enabled in OS X 10.9 or earlier, or if a firmware password is set, or if the startup volume is a software RAID, you can’t do this. Ask for further instructions.
Safe mode is much slower to start up and run than normal, with limited graphics performance, and some things won’t work at all, including sound output and Wi-Fi on certain models. The next normal startup may also be somewhat slow.
The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.
Test while in safe mode. Same problem?
After testing, restart as usual (not in safe mode) and verify that you still have the problem. Post the results of Steps 1 and 2.