My B747 autopilot no longer works correctely

[Guest jmferrer]

After a long period working perfectly, the autopilot of the B747 fails ramdomly in stabilizing flight parameters (altitude, attitude with respect to the flight director lines, etc) and I don't know what to do. Any one knows about this problem?. Thanks

[Guest fjbicrf]

Hmm.............. I seem to have a similar issue. I have flown several flights with both the 747-400 and 747-400F without issue. My flight yesterday evening in the 400F went fine until I was on approach. I was properly set up to intercept the glideslope, had three greens on the CMD buttons and Approach mode selected. When I intercepted the glideslope, the aircraft did not descend. I had to go around for another try at the approach but was unable to select the runway in the FMC. It was there but when I clicked it, nothing happened. I then tried to tune the ILS in the NAV/RAD page of the FMC. This did not work either. So, I ended up flying a manual approach - yikes!!!I do not have any failures selected on the PMDG menu.Hope that this was only an isolated incident.Fred Bray