Keep us posted as to whether it carries on behaving itself. If a channel is down and you're convinced it's been properly set up it still sounds to me like a cart problem, but you never know.

I think ESCo will examine and report on the state of a cart for not much money, so that might be worth doing for peace of mind.

In a day or two make a few fractional VTA adjustments (easy on an SME) if you still have a problem, see if that helps. Even tiny adjustments can make a surprising difference.

-- Ian