Thanx for all inputs, and for the first case "vehicle/aircraft distorting the ILS signal"; this is not what I'm talking about. Steady signals, just weird actions by A/P.
ASFKAP: Excactly, which was my original question. How can the A/P commands be different to the F/D commands? I have noted that older aircraft tend to have more pronounced errors like this.
Another thing that springs to mind is the abrupt motions on the control column when initially engaging the A/P in such aircraft. You sort of notice straight away that something's not quite right.
Speaking of TSF Venice Treviso with the unreliable GS. First you're kept high due terrain then you have to dive for your profile and then cleared ILS. So your roughly at 15-20 miles doing 250kts with sp.brk. and capturing LOC/GS. With a bit of luck the G/S signal moves up making the A/P pitch up quite abruptly due to the speed. I swear I could actually feel the G's doing this. I've seen the A/P disconnect lots of times, but it seems it just won't in the APP mode. I sort of get the feeling that said mode has different criterias to the others...Dunno, maybe it's just me...