Pitot/Static/Transponder Problem

Building to the plans doesn't guarantee accurate static pressure
readings; the system is quite sensitive to tiny plane-to-plane
variations. And the official calibration check with the airplane
stationary wouldn't catch an in-flight static-pressure error.

Have an observer watch the altimeter during takeoff and landing ground
rolls: if it changes from a standstill to takeoff speed and then
changes in the opposite sense during the landing roll (the VSI would
also depart from zero at these times), then you'll know that there's a
static-pressure measurement error built into your installation. The
amount of the spurious altitude change would be about 200 ft times the
ratio of takeoff/landing speed to cruise speed; e.g., if the speed
ratio is about 1/3, then the change would be 60-70 ft.

The static port(s) wouldn't necessarily have to be relocated if this
error is present; you may be able to tweak the readings sufficiently
with a wire-and-tape "trip strip" just ahead of the port (to lower the
static reading) or behind it (to raise the reading). Even some store-
bought airplanes have such cheap-and-easy fixes near their ports.
Just keep experimenting with the height and position of the strip
until the airplane doesn't "climb" or "descend" while trundling along
a level runway.