Yep, you have a head gasket leaking between 3 and 5, or a cracked head. Definitely take the heads in and have them pressure tested and resurfaced. Make sure you do both, not just the one. Replace the t-stat, while it's down, it's due. I believe while tracking this down, you tuned it up, so those parts should be ok to re-use. Change any filters you haven't already done, and make sure you have a good torque wrench for re-assembly.