Airspeed drop

I notice I fly 6-8 MPH faster, by GPS, doing the first 20 minutes or so of every flight. It's not wind direction or OAT. I think it must be the C/S governor and the oil finally reaching operating temp. Maybe instrumentation is off a little but I run square through out the possible settings without much effect. I doubt it would be frictional forces in an old engine building to the point of reduced HP slightly as it finally stablizes. Rich or lean has little effect either. Compression is lower on one cylinder at 72 pounds, all others 78/80. 775 hours on engine, just tuned up with plugs, wires, and rebuild mags. Any ideas?

The first thing to allow for is Altitude.
True airspeed increases with altitude. This will be recorded by the gps.
If I understand you right, what does mix clarity here is that your gps airspeed you report is dropping after 20 minutes of constant altitude flight. Does your Pitot and ASI confirm this rate of drop in Air Speed?