ACFT EQUIP PROB; XPONDER. FLC DISTR.

Narrative

CRUISING AT FL250 ENRTE FROM RIW TO DEN. CLRNC WAS DIRECT MBW VOR RAMMS 1 ARR DEN. WX WAS CAVU SMOOTH AIR. ABOUT 20 MI FROM MBW VOR (WHERE THE COURSE CHANGED AND STAR BEGAN); ATC SAID WE WERE 500 FT LOW. WE VERIFIED BOTH ALTIMETERS WERE WITHIN 100 FT OF FL250 AND BOTH ALTIMETERS WERE SET TO 29.92. WE ADVISED ATC WE WERE MAINTAINING FL250. AT THIS TIME WE BEGAN TO TROUBLESHOOT. THE XPONDER(S) HAVE A TEST MODE WHICH SHOWS MODE; ALT BEING SENT TO ATC; SURE ENOUGH IT WAS SENDING BACK FL245 EVEN THOUGH WE WERE MAINTAINING FL250 WE TRIED SWITCHING XPONDERS; NO HELP; STATIC HEAT NO HELP; RECYCLING NO HELP; ALTERNATE STATIC. FINALLY REQUESTED DSCNT TO FL230 TO SEE IF THIS WOULD HELP. WE HAD JUST COMPLETED THE DSCNT CHKLIST WHEN WE NOTICED WE HAD CROSSED MBW VOR WHERE WE SHOULD HAVE TURNED. WE OVERSHOT BY 8 MI. WE BEGAN AN IMMEDIATE TURN BACK ON COURSE. ATC THEN ADVISED US WE WERE 10 E OFF COURSE. WE OFFERED OUR APOLOGIES AND SAID WE WERE CORRECTING. THERE WAS NO TERRAIN OR TFC CONFLICT (WE WERE VMC). IN 4000 HRS OF FLYING I HAVE NEVER DONE ANYTHING LIKE THIS NOR HAD THE CAPT. OBVIOUSLY WHAT CAUSED IT WAS PREOCCUPATION WITH THE XPONDER. I AM WELL AWARE OF THE PROBS DISTRS CAN LEAD TO. IN FACT; I USED TO TEACH MY STUDENTS NOT TO LET MINOR DISTRS INTERFERE WITH TASK #1; FLYING THE AIRPLANE! PERHAPS I NEEDED A REFRESHER IN THAT SAME LESSON AND TODAY I GOT IT. MY ADVICE TO FELLOW PLTS; WHEN A PROB (DISTR) OCCURS; DEAL WITH IT AND THEN WITHOUT DELAY GET BACK TO FLYING THE AIRPLANE!