Were you able to reproduce the problem. My PMDG- 747-400 is doing a similar thing. It will not follow the flight director in a similar way as you described and then bust through the cruise altitude and keep climbing. Pressing FLCH starts it to return to the FMC and MCP cruise altitude setting, and then it can just just climbing again on its own. I also found that V/S set to negative (decent) and it will continue to climb. Further along in the flight it seems to settle down and start responding holding VNAV and following altitude commands as it should. There has been a few (but rare) posts but no resolution I have found so far I have seen. I have also noticed on the stats page the elevator right up one end, as you described, only by switching off AP, does the elevator return to normal, but you have to re-trim a lot after disconnecting the AP. As mentioned in other forums I used the PMDG load manager first.