MD80 DEPARTING DFW ON TRISS RNAV SID DEVIATED N OF COURSE ON TRANSITION PORTION.

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OUR CLRNC WAS THE TRISS 2 DEP FROM DFW. THE WAYPOINTS WERE CHKED AT THE GATE. WE TOOK OFF ON RWY 35L. AT 500 FT WE ENGAGED RNAV TRACK. AT ABOUT 100 FT AFL; THE AUTOPLT WAS ENGAGED. THE AUTOPLT/FMS TRACKED THE RNAV DEP NORMALLY UNTIL IN THE VICINITY OF SHERO INTXN. AT THAT POINT; CTR SHOWED THE ACFT ABOUT 4 NM N OF COURSE. THE MAP DISPLAY SHOWED THE ACFT ON CTRLINE. A SWITCH TO TXK VOR (RAW DATA) SHOWED US ONE DOT L OF COURSE AT ABOUT 75 NM W OF TXK. AT THAT POINT WE USED VOR NAVIGATION FOR THE REMAINDER OF THE RNAV DEP. WE INFORMED ATC OF THIS. LATER; PASSING OVER TXK; THE FMS SHOWED WITHIN A MILE OR SO. I REALLY CAN'T ACCOUNT FOR THE DRIFT OFF COURSE. THE FMS SHOWED TRACKING CTRLINE. I ALWAYS CHK THE ACFT SYMBOL AT THE END OF THE RWY WHEN WE GO IN POSITION. I KNOW THE FMS STARTED OUT IN THE RIGHT POSITION. NO ONE MADE ANY INPUTS TO THE MANAGEMENT UNIT AFTER TKOF. THE FMS WASN'T IN DR MODE. WE WERE PICKING UP AN INCREASING WIND OFF THE L WING. EITHER THE DRIFT WAS PERHAPS CAUSED BY SOME ELECTRONIC INTERFERENCE FROM THE CABIN OR WAS ANOTHER MD80 ANOMALY. I HAVE SEEN RTE DISCONTINUITIES MYSTERIOUSLY SHOW UP WITH ELECTRICAL POWER SHIFTS AFTER R ENG STARTS; BUT NOTHING LIKE THIS. I LIKE TO USE RAW DATA FROM VORS TO CROSS CHK THE RNAV. HOWEVER; AT THE TIME ATC CALLED WE WERE BOTH STILL IN MAP MODE. I THINK FROM NOW ON I'LL IDENTIFY THE FIRST FIX ON THE RNAV DEP THAT CAN BE IDENTIFIED OFF A VOR; AND FROM THAT POINT ON SOMEONE WILL BE DISPLAYING RAW DATA.