DEPARTING TEB CESSNA 650 FLC AFTER A CLRNC BUST AND MODE C DIFFICULTIES GNDS THE ACFT.

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DEPARTED TETERBORO; NJ; ON 'TETERBORO 4 DEP.' THE SID CALLS FOR MAINTAINING AN ALT OF 1500 FT UNTIL APPROX 4 MI FROM TEB; THEN CLB TO 3000 FT. WHILE WE THOUGHT WE WERE COMPLYING WITH SID; THE CTLR QUESTIONED OUR ALT. HE THEN ADVISED US TO IMMEDIATELY CLB TO 4000 FT TO CLR TFC; WHICH WE DID. WE WERE OBSERVING 'VAGARIES' ON THE VOR SIGNAL (SWEEPING OF HSI) AND THOUGHT; PERHAPS; OUR DME LED US ASTRAY BY INDICATING APPROX 4 DME WHEN WE WERE NOT ACTUALLY AT THAT SPOT. ENRTE WE COMPARED DME DISTANCES AND VOR RADIALS AND FOUND NO DISCREPANCY. THE NEXT DAY; ENRTE TTN-UGN WE WERE TOLD BY ATC THAT OUR MODE C INDICATED 300 FT HIGH. WE ADJUSTED OUR ALT ACCORDINGLY AND EACH TIME WE WERE GIVEN AN ALT OR FREQ CHANGE; QUERIED THE CTLR AS TO WHAT ALT HE SHOWED FOR US. UPON ARR AT UGN WE 'GNDED' THE ACFT; THUS CANCELING ALL TRIPS; UNTIL THE FAULT COULD BE LOCATED. OUR MAINT PERSONNEL FOUND A DEFECTIVE AIR DATA COMPUTER AND REPLACED IT. AS A PRECAUTION; WE ALSO HAD THEM CHK AND SWAP VOR/DME RECEIVER EQUIP UNTIL IT CAN BE ASCERTAINED IF THIS COULD HAVE CONTRIBUTED. THEY DID DISCOVER A 300 FT ALT DISCREPANCY; SO I SUSPECT THIS CAUSED THE ERROR IN THE FIRST PLACE.