A B757-200; UNABLE TO MAINTAIN FL390 WITH A FAULTY EPR SENSOR; HAS TO DSND TO FL350 AND THEN DIVERT TO DFW; TX.

Narrative

ENRTE FROM MIA TO LAX NEARLY ABEAM MOB; WE REQUESTED CLB TO FL390 FROM FL350 TO MINIMIZE EFFECTS OF TSTM LINE PENETRATION. ACFT CLBED TO FL387 THEN LEVELED OFF. SHORTLY THEREAFTER; AIRSPD DECAY OCCURRED AND WE NOTED ERRATIC EPR INDICATIONS AND DISINGENUOUS COMPARATIVE ENG INST INDICATIONS -- THROTTLES REMAINED ALIGNED. THRUST APPEARED INADEQUATE TO MAINTAIN ALT SO WITH CLRNC WE DSNDED TO FL350. LEVELING OFF THERE; ALL L ENG INDICATIONS SHOWED HIGH THRUST WHILE R ENG EPR WAS PEGGED AT MAX; YET OTHER PARAMETERS SHOWED REDUCED THRUST. NO EICAS ALERTS OF ANY KIND EVER APPEARED. ACFT SPD WAS LESS THAN NORMAL FOR FL350 AND WOULD NOT INCREASE. MAINT WAS CONSULTED VIA PHONE PATCH ALONG WITH DISPATCH; GIVEN OUR READINGS AND DETERMINED AN EPR PROB WAS LIMITING NORMAL THRUST ON R ENG. SUGGESTED MATCHING ALL INSTS DISREGARDING EPR; THAT RESULTED IN 4 INCH THROTTLE MISALIGNMENT. DETERMINATION WAS MADE TO LAND AT DFW 200 MI AHEAD ENRTE FOR MECHANICAL INSPECTION. DURING DSCNT AND WITHIN A PERIOD OF TIME; ENGS EVENTUALLY RETURNED TO NORMAL AND WORKED THAT WAY THROUGHOUT THE REMAINING DSCNT; APCH AND LNDG. DISPATCH WAS NOTIFIED OF THE DIVERSION. CREW DID NOT DECLARE AN EMER; HOWEVER; ATC (ZFW) DECLARED EMER FOR US IN GIVING US EXPEDITED CLRNC TO DFW.