It is possible that the head gasket is leaking. What I have noticed over time is that a majority of technicians will use a "cookie cutter" pad to clean the cylinder head mating surface prior to reinstalling the head. I also noticed that some technicians are not completely satisfied with the surface unless it is shinny! With an aluminum head it takes little effort to actually remove material, a couple of thousands here or there when using a rotating disc is not something that can be easily detected. When cleaning the surface I believe some technicians have a tendency to become focused on removing “stains” (which can be .003 to .007 deep into the material) rather than the material which effects the sealing quality of the mating surfaces. I have encountered a number of situations now where the only solution was to remove the cylinder head, disassemble the valve train as needed, and have the cylinder head machined. I had one head that had to be replaced due to the surface being badly damaged from an over zealous cookie cutter operator.

1) The fact that your engine runs warmer on the highway implies that it is getting hotter while driving.
2) The fact that it is cooling down in traffic implies that it is capable of cooling itself sufficiently.

I just went through this uncomfortable process with a friends car, the cylinder needed to be machined due to the imperfections and now the engine temp is rock steady at 85-C. Valve job performed within the last 2 years………….IMHO I would encourage switching to a wood scraper (finishing scraper) when cleaning the cylinder head surface, it is faster.