Since I am running the motor in governor mode I would expect thrust to increase with lower ambient temperatures if RPM are kept constant.  But I would also expect drag to increase as well.  In my mind I figured that the two would even out and that the lap time would not change.

Exactly. However if you check the formula for lift and drag, you will see that they are linear not only to air density but also to lift and drag coefficients. And they will change in cold air. The reason is that cold air makes Reynold number lower. That means (if you check airfoil polar) that lift coefficient is lowering and drag coefficient increasing in cold air. And what is even worse, also critical RE number goes down in cold air, that makes larger part of prop run in overcritical RE number and so it will pull even better.

There is also another thing - typically thin air will make "vacuum feeling" and many flyers like to fly shorter laps in hot or thin air.

And yes, change of lap time is normal, top flyers on large contests practicing early morning and flying contest flights in hot afternoon measure ambient temperature (or also altitude if model is sensitive to that) and adjust RPM regarding temperature (or also altitude). We have program for smartphones for that 

It is tuned for my timer, but all those parameters allow to set it for any timer or ESC.

https://www.facebook.com/363984487112995/photos/a.364101993767911/650808675097240/

also for altitude

https://www.facebook.com/363984487112995/photos/a.364101993767911/653600621484712/



   

Bill,
To add to what Igor said you are picking up about 3.8 - 4% additional pitch efficiency in the colder temperature. If you want to keep the 5.4 lap time you could either reduce the rpm by 4% or decrease the pitch by same (this is a rough starting point and you can fine tune to your ship) or add ~4% to the line length. Some flyers have several sets of lines or different props setup with these small pitch changes (like 0.25"  i.e. 6" to 5.75") to allow the power plant to remain relatively constant as temps change, this works really good with electric as the rpm will be constant.

Best,   DennisT