This is very strange indeed. 

Some follow on questions: 
- Do you have any custom CSS on the page? 
- What are the contents of the tabs in question?  If you strip the contents out of the tabs (Move the contents below the tabset) does it still misbehave? 

Would you mind giving us Login Rights so we can look? 

I am using custom CSS – but even with that removed I’m still getting the issue.
As far as content, I’ve tried field editors, tables & no content - no difference.

Login access granted!

Pretty strange.  We’ll look into it deeper in the morning.  The salesforce sidebar has a “colapsed” div that is getting an extra 20 pixels injected into it when you are on the second tab.  If you remove the sidebar the problem will certainly go away.  (that is a setting in the page properties)  See below: 

Hi Pat,

I played around with this a bit and was able to recreate this in one of my orgs.  It looks like Salesforce’s adjustable sidebar is causing this extra padding.  I added this CSS rule to my page and it seemed to fix the issue.

.sidebarCollapsed {&nbsp; &nbsp; <br>&nbsp; &nbsp; padding:0;<br>}

Awesome - Thanks, Ben.
This does seem to work  – sitting a bit closer to the sidebar than before, but that’s a reasonable trade-off versus having the tabset shifting around the page.