Image position not set on page load - jumps on first scroll

I had parallax working nicely for a couple of months, until very recently (last day). Now, the image jumps horribly on first scroll. It seems to be because the placement is not being set correctly on page load. I have been doing some work today, so I reverted to pre-problem files (nodes, CSS, etc) but the problem remains.

I did do a routine module update this morning, and there were quite a few of them. I don't recall exactly what they all were, but all processed without error except Secure Pages which seems to be missing an update flag so even though the update has run, it continues to ask to update. I have not updated Drupal core for some weeks: it's on the latest.

I've uninstalled and reinstalled the parallax_bg module and poked around for many hours, but have not found a particular problem.

I tried to add some javascript code--to call the script on page load--to one of my pages but since I'm not a JS programmer, haven't been able to get the code right. Page here: http://www.catalyst2action.com/products/research-squirrel/research-plan

Is there a known module clash that needs fixing? Is it possible to at least have some inline JS code that can be inserted into the page so the parallax function is called on page load, until a proper fix is found?

Many thanks in advance.