Long CX IFR at 7000
Controller comes on "say Altitude" My response "7000" He says you show 300
feet lower and gives me the altimer reading. I confirm. He says "turn off you
mod C"
I comply.Look at my GPS which is at 7060.
I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
7060.
The controller comes back and says" I think this is due to a cold front in my
flight area.
15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod
C. He concurs my altitude at 7000.
Request lower for decend for landing. The gps works fine now, ATC is happy.

HAS ANYONE ELSE EVER EXPERINCED THIS?