Your T/O flow is wrong. however I doubt that along would cause real aircraft to overspeed. I thinke Real Boeing would have more thought about how the AP would behave when mishandled.

Try the proper flow to see if that works:
1.For 75/67, VNAV should NOT be armd on ground for TO
2.TO/GA button on TMSP should be select on pre-flight flow even before doors closed and engine started, but it's OK-ish to press here.
3.after lineup with runway and cleard for T/O, you should manually throttle up to about 1.1EPR, let the engine stable, and then press the EPR button above SPD button to engege AT.
  also, make sure you have turned F/D on on the ground, and the FMA shows       | TO | TO , before taking off.
4.Gears up, at Thrust reducing Altitude (above 400' at least, but normlly at 1000' , or 1500' for NADP), press VNAV or FLCH, the thrust mode should automaticly change to CLB, no need to press button on TMSP. lower the nose for speed up and retract flaps on schedule at accl alt.