I think the root cause may well be related to a timing issue but don't know where it might be. Your image size comment may help - I will check. Most of the pages are overlayed on a virtual aerial photo image which might be slower than ecw or other formats. I do this to be able to get the resolution I want as page is zoomed. I didn't have this issue 1 - 2 years ago when, excluding updates and such, everything was the same.
I didn't want to say this earlier but it seems like the loading of the site has a time limit set someplace. The site will load the map form but will not load some of the icon images , or the map itself. Then, if the map, or icon, is left clicked on it will usually finish loading. But frequently after this is completed the map will be presented off center and it is necessary to click the "size and center" icon. ??
My provider is Comcast, the only reasonable one to pick from in my area. Test speeds are: down - 88 mbps; up - 6 mbps. Still haven't identified how to test stability.
Appreciate you time and help!