On a site of mine I run adsense and have alternate ad set to display some links to other sites of mine. Lately it seems that sometimes I get neither the adsense ad or the alternate ad. I seem to have found a pattern though. It mostly happens when my browser has been open for a long time. If I am using a freshly rebooted computer or browser the ads show consistantly for a while before they start to fiz out. Clearing the cache on the browser seems to return things to normal. Could there be a caching problem with the JS that google uses to display ads? Has anyone else seen this happen?