In my 475A I was able to find what divider had the " worst" contacts, by swapping one at the time between Ch1 and Ch2.
I also soaked them one at the time in IPA and turned both the C set screws, to find the problematic one.
In the end, after cleaning, they were all ok, but had all dirty contacts.
As a test you could ground the output of the attenuator, just before the input of the amplifier.
Again, one channel at the time to see what happens.