javascript errors on upgrade from 3.6.9 to 3.8.6
 

I'm prepping for an upgrade to vb4 for a client, and have upgraded my test site to 3.8.6 from 3.6.9 as I have seen suggested. For the most part the custom templates are working, but I'm getting javascript errors in vbulletin_global.js?v=386 and vbulletin_menu.js?v=386:

Error: YAHOO is not defined
Source File: http://vb3.tagregator.com/clientscri...lobal.js?v=386
Line: 11
Error: vBulletin is undefined
Source File: http://vb3.tagregator.com/clientscri..._menu.js?v=386
Line: 11
Error: vBulletin is undefined
Source File: http://vb3.tagregator.com/clientscri...lobal.js?v=386
Line: 11

I do have mods in headerinclude and header, which work fine in the live forum at http://gm-volt.com/forum. I don't know if they're related to this issue, or even how to troubleshoot a javascript error of this sort, but it does seem that my test site is treating the menubar as though $show['popups'] is false, I assume because of the javascript errors.

Searching the forum didn't turn up anything of value. Also relatively new to vbull development, but plenty of experience in the related technologies, if that makes any difference.

Thanks in advance for any pointers!