Dumb question.
Could the randomness be explainable by the varying software at the ISP level?
Told you it was a dumb question, but if all other explanations fail, examine your premises.
There must be software from different sources available to ISPs, no? Maybe one "brand" interacts with SMF in certain circumstances to generate the problem: It loads the quote bar, then stops that process from the quote button, but not from the right-click, open in new tab process.
At least that would be an explanation as to why one lucky person and not another, and why some browsers and not others.
I'm not signing this one so nobody knows who the clown is.