You might check for leaks in the static system, or someone may have snuck a 'snubber' in the line, which can cause problems. We had a problem with the pitot static calibrations in one helicopter we were verifying when I was with Transport Canada - the pilot and copilot airspeed indicator had different corrections in climb and descent. Turns out one of the systems had a snubber installed and one didn't. The snubber is just a restricting device to slow down rate of change of static information. - You won't see any effect in straight and level flight....
Might be worth a look..