A BE400 FMS did not properly compute a crossing restriction. Situation went unnoticed until alerted by ATC who provided vectors off course and an amended clearance.

Narrative

While descending on the TWOLF1 STAR into FCM we were issued a clearance to cross TRGET Intersection at 8;000 MSL. I entered this crossing restriction into our FMS; monitored the FMS VNAV guidance; and began the descent at the FMS recommended vertical speed. The VNAV recommendation was initially 1;500 FT/minute which we were flying. Once descending through FL180 and approximately twenty miles from TRGET; I recognized that this descent profile would put us approximately ten to fifteen miles past TRGET when reaching 8;000 FT. I immediately began a maximum rate (non-emergency) descent and re-entered the same crossing restriction into the FMS. The new recommended rate was significantly different (5;700 FT/minute) than the original recommendation. This new rate would place us two miles past TRGET at 8;000 FT. ATC recognized and asked us about this trend. After confirming; the Controller turned us off of the arrival by sixty degrees. We crossed abeam (four miles offset) TRGET at 8;300 FT MSL. The flight continued to the destination without incident; no traffic conflicts resulted and no further explanation was requested by ATC. After speaking with the other pilot in the cockpit; he mentioned a similar VNAV discrepancy with the FMS two weeks prior to our event; but did not report his discrepancy. After completing our flight; we wrote up the FMS VNAV discrepancy in the MEL; however Maintenance could not duplicate the discrepancy and returned the aircraft to service.