parabellum,
Interesting you bring up FLCH. If they were in FLCH, a struggle involving the throttles would explain the altitude variations reported by military radar (the infamous F450 which was certainly not achieved). Pilots or hijackers try to grab the throttles, reset power and FLCH tries hold sped on pitch. Similarly, when the fuel gives out, plane descends at whatever speed was in cruise until impact, probably around 250 knots.