How to get the association lines to behave (and persist) in the EDMX designer?

Whenever I open my .EDMX file for editing, the association lines connecting the various entities are different (and varying wildly) from how I left them when I last closed it.

Here's VS2013's latest drunken spree:

I've tried editing the Diagram XML, but still I've found no success against this.

For example, here's the XML for the longest misbehavior above:

<AssociationConnector Association="Model.FK_InvoiceItems_Invoices" ManuallyRouted="true" >
  <ConnectorPoint PointX="2" PointY="11.875" />
  <ConnectorPoint PointX="6.5" PointY="11.875" />
</AssociationConnector>

It's pretty straightforward: we should be seeing just one straight horizontal line between Invoices and InvoiceItems. I have no idea how we ended up with this mess. I can drag it back into place, save it and close it, but as soon as I open the file up again it comes back.

Is anyone else having the same problem?

OK, I've decided that in its current iteration the EF designer simply isn't able to handle what I'm trying to throw at it. It appears to be a bit buggy yet.

So I've turned off ManuallyRouted in all elements and rearranged my Entities instead so that I don't get overlapping lines (which is what I was trying to accomplish in the first place).

There's far more important stuff in the world to worry about than this.

Thanks for your attention.