Yup... First thing I would do now is blow away the extension, build a dummy one, then rebuild the extension with a default COS, then delete the dummy one, and see if the problem persists, if it does replace the set, if it doesn't then change the COS back to what it was and if all good your all set, if the problem crops up again turn off COS features until you find the culprit!
The dummy extension is just a placeholder so you don't rebuild the phone with the same software ID, it's an old SX200 trick when you have strange feature issues.