[TxMt] Inconsistent selection with shift-option-down

[Chris Fonnesbeck]

I am trying to replicate the "Edit each line in selection" behavior that disappeared with the arrival of TM2. So, following the blog post at http://blog.macromates.com/2011/multiple-carets/, I place my cursor at the first line of a selection that I want to edit, and press shift-option-down. However, rather than selecting an additional line with every keystroke, it appears to jump arbitrarily between selecting single and multiple additional lines.

Here's a short video screen capture of the behavior: http://cl.ly/2q1x38141S1H3w0A0k2i

Thanks in advance for any help.
cf