Inaccurate airspeed indicator

I had a my two-year transponder/static system check while visiting my mother
in San Antonio. When I departed, I noticed the airspeed didn't come up like
it should. I attributed this to shifting winds and assumed I had a slight
tailwind. Then in cruise, I did my normal operating performance check. My
IAS was about 30 knots low, but my groundspeed was normal. I tapped on the
airspeed indicator and it gained six knots. Question #1: How would a
transponder/static check screw up my airspeed? Was this just coincidence?
Question #2: How do you check accuracy of an airspeed indicator? Question
#3: How long should you go before replacing an airspeed indicator?