Just found the cause of the problem after doing some searches on "ga_shade" It appears to be a bug connected with a cookies set when you use the Google Analitics page overlay to see the number of clicks on various links on the page. It can be resolved by deleting the cookie. In my case, replacing the GA code after a few days I found everything to be fine (I assume the cookie had died by then)
The good news also is that it is only the owner of the GA account that sees this effect!
Would any mods like to move this back to the Analytics forum where I think it belongs?