FAULTY MODE C ON ACR Y LEADS TO ERRONEOUS TCASII RA FOR ACR X.

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OUR FLT ACR X AT FL260 ENRTE OKK VOR; 110 MI S ON 170 DEG RADIAL. WE OBSERVED A TCASII ADVISORY OF TFC AT 12 O'CLOCK; 1000 FT ABOVE AT ABOUT 15 MI ON AN OPPOSING HDG. SHORTLY AFTER WE OBSERVED TARGET ON TCASII DISPLAY; DSND FROM 1000 FT ABOVE TO 500 FT ABOVE. TCASII COMMANDED A DSCNT OF AT LEAST 2000 FPM TO AVOID TFC. WE QUERIED ZID OF TFC. THEY TOLD US WE HAD AN ACR Y JET 1000 FT BELOW US ON A CONVERGING HDG. AT ABOUT THE SAME TIME WE VISUALLY ACQUIRED ACR Y ABOUT 500 TO 1000 FT BELOW OUR ALT. CTR ALSO CONFIRMED HE WAS ASSIGNED FL250. WE OBSERVED NO TFC ABOVE NOR DID CTR HAVE ANY TFC ABOVE US. OUR TCASII CONTINUED TO COMMAND A DSCNT AND CONTINUED TO SHOW OUR TFC CONFLICT 500 FT ABOVE US. CTR ADVISED US THAT ACR Y'S MODE C DID MOMENTARILY SHOW '26500' AND THEN RETURNED TO 250 ON THEIR SCOPE. WE HAD ALTERED COURSE SLIGHTLY TO THE R TO OFFSET CONFLICT BUT DID NOT FOLLOW TCASII RA. IF WE HAD FOLLOWED TCASII 'RA' WE; IN MY OPINION; WOULD HAVE IMPACTED THE OPPOSING ACFT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THAT THE RPTR HAD SOME AMBIVALENT FEELINGS ABOUT TCASII; AND FEELS THAT THERE IS A LONG WAY TO GO FOR SYS MATURITY. SUPPLEMENTAL INFO FROM ACN 210600: WE WERE BEING STEP CLBED TO OUR CRUISE ALT AND PRESENTLY CLBING TO LEVEL AT 25000 FT. IT WAS THE CAPT'S LEG SO WE WERE OPERATING ON #1 ATC TRANSPONDER #1 ALT RPTING (MODE C). ZID CALLED TFC AT 12 O'CLOCK ABOUT 12 MI AT 26000 FT. I; FO; SPOTTED TFC ABOUT 7-8 MI OUT. THEN WE HEARD THE ACR X JET RPT A TCASII ADVISORY SAYING THE GUY WAS SHOWING 500 FT ABOVE THEM. OUR ACFT WAS NOT TCASII EQUIPPED. I CONTINUED TO OBSERVE OUR TFC AS HE PASSED OVER US AND IN A SLIGHT R-HAND BANK. HE APPEARED CLOSER THAN THE STANDARD 1000 FT SEPARATION. ATC THEN TOLD US OUR MODE C SHOWED 26500. THE CAPT AND I BOTH VERIFIED 29.92 AND 25000 ON OUR ALTIMETERS. THE WHOLE SITUATION BECAME VERY CLR. ACR X GOT A TCASII RA SHOWING US AT 26500 FT TELLING HIM TO DSND FROM 26000 -- WHILE WE WERE ACTUALLY AT 25000 WITH A FAULTY MODE C WHICH PLTS HAVE NO WAY OF MONITORING. (IT'S INTERESTING THOUGH; THAT FOR BEING STEP CLBED FOR 25000 FT; NOT A SINGLE ATC CTLR INFORMED US OR NOTICED OUR INCORRECT ALT RPTING MODE C.) WE INFORMED ATC AND SWITCHED TO OUR #2 TRANSPONDER AND #2 ALT RPTING (MODE C). THEN ATC ADVISED US OUR MODE C SHOWED THE CORRECT ALT OF 25000 FT. LATER WITH ZTL WE ASKED HIM TO VERIFY BOTH #1 AND #2 MODE C'S -- HE SAID #1 SHOWED 1500 TO 1600 FT HIGH; BUT #2 WAS RIGHT ON OUR ASSIGNED ALT. NOTE THAT THIS PROBLEM HAD BEEN WRITTEN UP IN THE MAINT LOGBOOK TWICE IN THE PAST WK. THIS COULD HAVE BEEN A POTENTIALLY FATAL SITUATION HAD IT NOT BEEN VISUAL CONDITIONS.