ACFT WAS FLYING 3000 FT HIGHER THAN INDICATED BY 2 OF THEIR 3 ALTIMETERS AND THEIR MODE C.

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ON CLBOUT; A SPLIT WAS NOTICED BTWN ALTIMETERS. ONE SHOWED HIGHER THAN OTHER 2. ATC VERIFIED OUR ALT READOUT AND IT MATCHED 2 OF OUR ALTIMETERS. UPON LEVELING AT FL290; TFC WAS CALLED AT 12 O'CLOCK; 3 MI AT FL310. THAT ACFT PASSED BELOW US. ATC WAS ADVISED BY ME OF THIS SIT. THE OTHER ACFT SAID THAT TCASII SHOWED US 2000 FT BELOW THEM; BUT IN FACT WE PASSED ABOVE THEM. ATC STILL SHOWED US AT FL290. FOR THE NEXT 3 HRS; EACH CTLR SHOWED US AT FL290 AND 3 OTHER ACFT VISUALLY SAW OUR ACFT ABOVE THEM WHILE THEIR TCASII SHOWED US BELOW THEM. EVEN AFTER SEVERAL ACFT HAD VISUALLY CALLED US HIGHER THAN FL290. ATC REQUESTED WE TRY ANOTHER XPONDER AND TO VERIFY 29.92 SET IN OUR ALTIMETERS. IT APPEARS THAT TCASII MUST RECEIVE ITS INFO FROM XMISSIONS SENT OUT BY ENCODING ALTIMETERS. OUR ENCODING ALTIMETER WAS SHOWING US AT FL290. ATC WAS PAINTING US AT FL290; AND SEVERAL TCASII EQUIPPED ACFT SHOWED US AT FL290. THE ACFT WAS; AS WE CAN BEST DETERMINE; AT FL320. HAD THIS NOT BEEN A CLR NIGHT; AND WE WERE NOT ABLE TO VISUALLY SEE OTHER ACFT; THIS COULD HAVE CREATED A VERY DANGEROUS SIT. THE PROB WAS FOUND TO BE A LOOSE/CRACKED PITOT TUBE. IF TCASII IS USED AS A MEANS OF COLLISION AVOIDANCE; THEN IT MUST BE REQUIRED TO PICK UP TARGETS BY OTHER MEANS THAN THE SIGNAL SENT OUT FROM ENCODING ALTIMETERS. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: AT CRUISE; THE CAPT'S ALTIMETER SHOWED 32000 FT BUT BOTH ALTIMETERS ON THE FO'S SIDE PLUS ATC MODE C READOUT SHOWED THE ACFT AT 29000 FT. THEY CYCLED THROUGH ALL THE STATIC SOURCES AND STILL COULD NOT BE SURE WHICH ALTIMETER(S) WERE IN ERROR. ATC HAD OTHER ACFT PASSING CLOSE ABEAM AT 31000 FT AND ASKED THOSE ACFT WHAT THEY SAW. THREE OF THEM SAID THEIR TCASII SHOWED THE RPTR'S ACFT AT 29000 FT BUT THAT IT APPEARED TO BE HIGHER THAN 31000 FT. THE CREW DETERMINED THEY WOULD ONLY LAND AT AN ARPT WITH VFR WX. AS THEY DSNDED; THE ALTIMETER SPREAD DECREASED UNTIL; AT LOM PASSAGE; THE SPREAD WAS WITHIN ALLOWABLE LIMITS. LATER INVESTIGATION SHOWED THAT BOTH ALTIMETERS ON THE FO PANEL WERE CONNECTED TO THE SAME STATIC SOURCE AND SHOULD ONLY BE CONSIDERED AS ONE ALTIMETER IN THIS CASE. HE SAID THAT THE CABIN ALT AND PRESSURE DIFFERENTIAL READOUTS WERE INCONCLUSIVE IN SOLVING THE PROB. HE PRAISED THE ATC HANDLING THEY RECEIVED.