Autopilot and ILS crashes 747

Hi,

I'm relatively new to FS2004, and I'm just trying to learn to land the
747 using ILS and autopilot.

I've set up a flight where I'm just about to capture the glide path at
2000ft at Sydney airport. I have no problem capturing the glide path.
Alt switches off on the autopilot when I intercept. However the
autopilot was crashing the 747 on landing. It was bobbing the aircraft
up and down slightly, which I believe was causing the problem. I started
speeds from 140 knots to 200 in increments of 10. 140 falls out of the
sky. 170 crashes, 150 is perfect (no crash).

Is this a bug or am I missing something vital? Does a real ILS behave
this way? Is there an easier way to determine the correct speed?

Forgive my ignorance if I'm missing something obvious

Thanks,

Sammy