What was found
PILOT'S FIRST REPORT, AFTER START, SEC INV LIGHT CAME ON. RESET THE SWITCH TWICE & THE LIGHT WENT AWAY. FOUND LOWER CONNECTOR UNSEATED. CLEANED CONNECTOR & RESEATED. OPERATIONS CHECKED GOOD. PILOT'S SECOND REPORT, AFTER TAKEOFF AT FL 350, ACFT SPIKED TO 140 - 150 VOLTS & A SUDDEN CHANGE IN AUTOPILOT PITCH. TURNED SEC INV OFF, VOLTAGE WENT BACK TO 120V, INV LIGHT CAME ON. R & R SECONDARY INVERTER, OPERATIONS CHECKED GOOD. AUTOPILOT ALSO CHECKED GOOD. NO REPEATS AFTER INVERTER REPLACEMENT.