Boeing 707-420 ILS problem

When the Boeing 707-420 is locked to the GS and descending at a normal rate of 750 FPS, all is going well until the aircraft gets to within a few miles of the runway and then all of a sudden it will nose up for a couple of hundred feet and then come back down on the GS. It will do this every so often on the GS for no apparent reason which means if you want to land properly you will have to disengage the GS otherwise it will nose up at the wrong time and overshoot the runway. The file I am using is b7074v22.zip but I believe there are several variations of this aircraft. Wonder if anyone else has experienced this nose up anomaly. It also has a tendancy to nose up and down during flight as if to hit turbulence even though the settings are for calm, clear weather.

Regarding the nose up tendency during approach with autopilot I think it has to do with the glide path indicator. I've not flown an ILS in real life yet so I don't know if it happens for real but in in the FS when I'm manually doing an ILS and at about 0.5 NM from the runway I notice suddenly the GPI rises (i.e. indicating you are falling too low) in spite of the PAPI saying that you are correct on the glide slope. I usually just disregard this indication and maintain a constant rate of descent and arrive just perfect on the runway.

I believe this is to do with the narrowing of the glide path signal as you get closer to the transmitter. As in large aircraft the flight path and the GP antenna's path do not coincide, as the beam getts narrower it may be that the GP antenna is situated below the angle where the indication would be "on the glide slope". I may be wrong on this, however.

Hi and thanks for the reply. I should have stated that this only seems to happen on the 707-420 as all of the other aircraft be it fighters or very large passenger jets come in smooth as glass. It appears that this problem is only on this particular aircraft so my thinking is that is is built into the software and probably cannot be avoided. Be it as it may, this aircraft is one of the best ones to fly that I have as you really have to keep your eye on the airspeed indicator as in the blink of an eye it can stall or overspeed rapidly. I wonder if John Travolta has any problems like this? I doubt it or it would be heard loud and clear. I often wondered why he purchased a 707 to begin with. Must have gotten it at a good price. Thanks once again.