What was found
FLIGHT NR 3983, ON TAKEOFF, APPROX 700 FT, AUTOPILOT ENGAGED, FLAPS 8, RECEIVED STAB TRIM CAUTION MSG FOLLOWED BY MACH TRIM CAUTION. THEN RECEIVED AUTOPILOT TRIM CAUTION MSG. DISCONNECTED AUTOPILOT, FOLLOWED AN ABRUPT NOSE UP PITCH CHANGE. COMPLIED WITH QRH, REENGAGED STAB CH 2 & REGAINED NORMAL CONTROL OF ACFT. ACTION TAKEN IN PROGRESS.