FLT CREW OF B737-300 FIND THE LNAV MODE OF THEIR FMS FAILS TO PROVIDE PROPER GUIDANCE ON SID OUT OF PDX. REVERT TO RAW DATA TO MONITOR RADIAL INTERCEPT.

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I WAS THE PLT MONITORING; THE FO WAS THE PF. WE TOOK OFF RWY 10R AND FLEW RIVER 6 DEP. THE FO DID A GREAT JOB FLYING THE DEP ON RAW DATA (VOR/ILS). WHILE TRACKING OUTBOUND ON THE PDX 085 DEG RADIAL; ATC GAVE US A VECTOR OF 030 DEGS TO INTERCEPT J-16. THE FO TURNED TO THE HDG AND I RESET BOTH VOR CTL HEADS TO THE BATTLEGROUND (BTG) FREQ AND PUT THE 069 DEG COURSE IN BOTH OF OUR COURSE WINDOWS. I XCHKED MY MAP AND VERIFIED THAT THE LNAV DATA WAS CORRECT AND NOTIFIED THE FO THAT LNAV WAS GOOD. HE THEN SWITCHED TO LNAV WHILE I CONTINUED TO MONITOR THE INTERCEPT OFF THE RAW DATA. THE LNAV GUIDANCE APPEARED TO START OUR TURN EARLY; I ASSUMED THE CDI (ON MY SIDE) HAD NOT STARTED TO MOVE BECAUSE OF HOW CLOSE WE WERE TO THE VOR. ONCE THE FO HAD INTERCEPTED THE LNAV COURSE; IT WAS APPARENT THAT WE WERE R OF COURSE LOOKING AT THE RAW DATA ON MY SIDE; SO I DIRECTED THE FO TO COME BACK TO THE L AND TO INTERCEPT OFF THE RAW DATA. THE FO COMPLIED AND RETURNED TO THE VOR/ILS MODE. THE CTLR DIRECTED US TO COME BACK TO THE L AND WE TOLD HIM WE WERE ALREADY IN THE TURN. THE CTLR REQUESTED A CALL WHEN WE LANDED. WHEN WE LANDED WE CONTACTED THE CTLR AND HE SAID THERE WAS A PROB EVERY DAY WITH THIS COMPANY FLT TURNING EARLY ON THAT INTERCEPT; AND SAID HE HAD A TRAINING CTLR AND FOREWARNED HER THAT WE WOULD BE TURNING EARLY. HE ALSO STATED THAT ANOTHER CARRIER B757 HAD THIS PROB AND IT HAD SOMETHING TO DO WITH THEIR DATABASE IN THE ACFT. HE ASKED ME TO NOTIFY A CHIEF PLT SO WE COULD TRY TO REMEDY THE PROB. IN THE FUTURE; UNLESS I AM CLRED DIRECT TO A FIX IN THE -300/-500; I WILL BE FLYING ONLY ON RAW DATA. IT APPEARS WITHOUT A VISUAL DEPICTION OF YOUR GND TRACK YOU CANNOT BE 100% SURE OF AN INTERCEPT CLOSE TO A VOR.