What was found
AT 390 BOTH ENGINES HUNG AT .7 EPR AFTER MOUNTAIN WAVE EVEN THOUGH THROTTLES WERE MANUALLY ADVANCED, DID NOT SPOOL UP FOR SEVERAL SECONDS LOST 10 KTS AIRSPEED. *S/D* THERE WERE NO FAULT MESSAGES, AND BOTH ENGINES PASSED A HIGH POWER RUNUP AND ACCELERATION TEST ON THE GROUND. THE AIRCRAFT WAS RETURNED TO SERVICE WITH NO SUBSEQUENT GRIPES OF SLOW ENGINE SPOOL UP AT ALTITUDE. SINCE THERE WAS NO EICAS ENG THRUST ANNUNCIATION DURING THE EVENT, THE ENGINE ACCELERATION RATES DID NOT EXCEED LIMITS. NO CORRECTIVE ACTION IS REQUIRED.