It may be happening sporadically to others, but, not at the same time, if my theory is correct. I suspect that specific advertisements are responsible and these ads are targeted at specific people according to their cookies and ISP location. As evidence, I bet nobody here but me sees the following ad:
Although this particular ad. is not a culprit. Often, when the long running script message appears, there is a whirly icon (the thing that replaced the hour glass icon) inside the ad. box.