vs2008 website upgraded to vs2013 pulling wrong system.core

I have two sites that are vs2008, .net 3.5. I am upgrading them to vs2013, but want to keep them at .net 3.5. I carried out the upgrade and one site runs fine as .net 3.5, but the other one gets this error message:

Could not load file or assembly 'System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

Post-policy reference: System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089

Using fuslog shows the .net 3.5 system.core loading correctly. But immediately afterwards it tries to load system.core 4.0.0.0 with the calling assembly shown as:

Calling assembly : (Unknown).

I've already checked to make sure every project in the solution is targeting .net 3.5 I can't figure out why the other site works fine - their references are identical.