What was found
CLT FLT 1042 EN-ROUTE BWI TO CLT. STAB TRIM EICAS MESSAGE ON CLIMB. EICAS MESSAGE WENT AWAY AFTER 45 SEC. 60 SEC LATER, WITH A/P ENGAGED AT FL280, A/C PITCHED DOWN. CREW FELT BUMP. 300-400 FPM DESCENT STARTED THEN STOPPED. ON APPROACH, AC PITCHED DOWN WITHOUT A/P ON. HAPPENED THREE TIMES WITH HEAVY NOSE DOWN PRESSURE. MAINTENANCE ACCOMPLISHED HORIZ AND STAB TRIM TEST, STAB TRIM MESSAGE MAN MODE, & ELEV CONTROL SYSTEM TEST. ALL CHECKS OK.