A DC8 FREIGHTER DIVERTS TO AN ALTERNATE ARPT AFTER MALFUNCTIONS APPEAR IN THE 3 ALTIMETERS AND AIRSPD INDICATORS.

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AT CRUISE AT FL350 APPROX 1 HR INTO FLT FO'S ALTIMETER BEGAN FLUCTUATING. AS FO ALTIMETER REACHED 400 FT LOW; CTL OF AUTOPLT WAS SWITCHED TO THE CAPT'S SIDE. CAPT'S ALTIMETER SHOWED STILL LEVEL AT FL350. IT WAS THEN NOTICED THAT THE STANDBY ALTIMETER WAS READING 300 FT HIGH. OVER THE COURSE OF THE NEXT 45 MINS THE FO'S ALTIMETER FLUCTUATED FROM 1000 FT TO +300 FT. STANDBY ALTIMETER CLBED TO +800 FT. AT THIS POINT WE WERE NOT SURE WHICH OF OUR 3 ALTIMETERS WAS CORRECT BUT WE BELIEVED IT TO BE THE CAPT'S. WE ADVISED ATC OF OUR PROB. THEN THE CAPT'S ALTIMETER BEGAN TO FLUCTUATE LIKE; BUT NOT IN THE SAME DIRECTION AS; THE FO'S. AT THIS POINT UNSURE OF WHICH ALTIMETER WAS RELIABLE BUT NOW BELIEVING THE STANDBY TO BE CORRECT AND IN VFR WX WE CHOSE TO DIVERT TO DSM. IT IS VERY CONFUSING TO HAVE 3 SEPARATE ALTIMETERS SPREAD BY 2000 FT (34000; 35000 AND 36000 FT). THE ACFT HAD BEEN DEICED WITH TYPE 1 AND ANTI-ICED WITH 100/90 TYPE 4 ULTRA PRIOR TO TKOF. IN ADDITION; AIRSPDS ON THE 3 AIR DATA COMPUTERS (RPTR MEANS 'ON THE 3 AIRSPD INDICATORS') WERE ALSO SEPARATED BY APPROX 30 KTS.