You are on the right track-check the internal connections at the offending binding post or all of them.
An excerpt from this thread:
"Every TIU I have opened up has had at least one nut on the binding post not tight enough. To fix this you need to get some M4-.70 nuts. Unscrew the nut on the binding post and remove the external washer and wire. Replace the nut and tighten. Then replace the wire. Next the external washer. Then the nut."