Any third party calls that may be hanging up? I know, that's such an obvious thing to look at first but I haven't seen it mentioned.
Is it possible that the order of scripts loading may be an issue at times? I've seen unusual things happen at times if the script references in the head are not in proper order.
Is it possible that there is a conflict in the scripts? And that at certain times, depending on how fast the page loads, certain scripts may load out of sequence or something?
Just guessing now based on stuff I've run into during my daily activities online. I've seen lots of issues with third party calls, CDNs, etc. Google included.