OK, you need to find out if the problem is in phonostage on in TT as the problem was “spanning several turntables and arms”. You might run your phonostage from anti-RIAA filter from your CD player:
or use one of many CDs with inverted RIAA curve :
if your phonostage out of suspicion the you can look at TT and arm but I kind of skeptic as you have the same problem with turntables and arms. If it was just “several arms” than I might suspect that your arm in not properly located at your TT but it if spread across several turntables then it is something else. You said that this only happens in the last 2 or so tracks on any given LP side that is a clear indication of geometry problem but why would you have the same problem across the several turntables and arms! It is dose not sound right to me.
The only explanation that I have in case if it were be not phonostage is that your dealer that “professionally setup” your TT was just an idiots and had no idea what he is doing. BTW, many of them are idiot and I do not insult then but just publicizing the sad fact. I know a few very experienced professionally setup specialists who actually cheat and verify arm geometry only by the protractors at a single. This is wrong. Well, it is OK if the arm was mounted at right spot but it very frequently not the case and you need two point of protractors alignment. At one point of middle the arm is aligned but at the inner-side of the record it is not aligned anymore and the needle slide sideways. With short arms and Shelter 901- like cartridges that has not cylindrical needles but rather the contemporary profile with shard sided the alignment shell be very precise. Otherwise you will be destroying you record (which is not a bid deal - it is Pink Floyd anyhow) and you might pick up all imaginary noises in the peripheral of record. If it is the case then your anti-skating shell run away very un-smoothly when your Shelter begin to position itself sidewise.
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche