Does it happen in safe mode (hold Shift when launching TB)? If it doesn't, the cause may be incompatible add-ons or broken layout. Some add-ons like Card Book or Signature Switch are suspect.

Thanks for your advice but because it only occurs randomly it's hard to know whether safe mode is a factor. The only add-ons are the default ones (I've not added any)

It may seem random, but probably isn't, e.g. does it happen when sending to mailing lists or just when the message has attachments? Safe mode applies a default layout, so you could see if launching in safe mode, checking 'Reset toolbars & controls' and 'Make changes & restart', makes a difference. Security apps that scan outgoing mail are also a potential source of problems.