ATC RADAR EQUIP PROB MODE C.

Narrative

ACR X AS WE WERE CLBING THROUGH APPROX FL176 (NOTE: THE ALTIMETER SETTING WAS QUITE HIGH THAT DAY; SO THE ALTIMETER AND ALT ALERTER WERE ALREADY SET TO 29.92) TO AN ASSIGNED ALT OF FL180; THE CTR QUERIED US AS TO OUR ALT. THE FO WAS FLYING; SO I RESPONDED THAT WE WERE 'ABOUT TO LEVEL AT FL180.' THE ATC CTLR THEN SAID EXCITEDLY THAT 'I SHOW YOU AT FL185;' AND ADMONISHED US TO 'SET YOUR ALTIMETERS.' AS I PREVIOUSLY RPTED; OUR ALTIMETER SETTINGS WERE ALREADY PROPERLY SET AT 29.92; AS WE WERE BY THEN OUT OF FL178 TO LEVEL AT FL180. NEEDLESS TO SAY; IT IS QUITE DISCONCERTING TO HAVE THE CTR PUT THE THOUGHT IN YOUR HEAD THAT YOUR PITOT STATIC SYS MAY BE MALFUNCTIONING. WHILE THE CTR THEN SAID; 'I NOW SHOW YOU LEVEL FL180;' HE DIDN'T OFFER AN EXPLANATION AS TO WHAT REALLY HAPPENED ON HIS END. LATER THAT DAY; I HAD THE APPROPRIATE DEPT WITHIN MY COMPANY RESEARCH THE OCCURRENCE. THEY HAVE SINCE INFORMED ME THAT ATC HAS ADMITTED THAT FOR 1 SWEEP; THEIR SCOPE ERRONEOUSLY INDICATED FL185 AND ON THE NEXT SWEEP SNAPPED BACK TO FL178. ALSO; I HAVE NOW BEEN INFORMED THAT THEIR EQUIP DOES ON OCCASION MAKE THIS TYPE OF ERROR. AS AN EXPERIENCED PLT; I CAN TELL YOU THAT WHEN THIS HAPPENS IT BECOMES A MATTER OF SAFETY; SINCE YOU ARE VERY DISTRACTED AS YOU TRY TO FIGURE OUT WHAT'S HAPPENING.