Certain head gaskets still need a second torqueing after a certain amount of hours. Many newer ones are torqued once at installation. Nevertheless I think you will find your problem in that area, as it was worked on most recently. Was the head checked for warpage when the gaskets were done? Your local auto mechanic will probably have a tester that samples your coolant for hydrocarbons. As previously stated, the oil pressure is much higher than coolant pressure and so the oil will show up in the coolant first. Check around with a few mechanics. I think you need to sample the coolant hot-ask the mechanic with the tester.
I am also in agreement that your engine is running a little cold. 180 degrees is generally considered the point at which your most efficient combustion and, more importantly, you begin to burn out moisture and impurities from the engine's lubricating system. A cold running engine also hastens the demise of everything down stream from the exhaust manifold.
Good luck. I'm bettin' on the head.