You may have fixed your problem in the short term, but I wonder about the long term effect on the N-Butyl rubber rings. They tend to last 10-15 years or more in normal lab environments. Did you inspect the contact pad on the attenuator sections, you might need a microscope (10x?), (particularly on the 10dB pad -smallest pad, near the input). I have seen divots on the pads, dimples on contacts where slight overpowering had caused contact welds transferring material from pad to contact, and anomalies in reading repeatability. I solved this issue with alcohol wetted paper pulled between the contact and pad, a mild abrasive. Then loosening the atten pad, repositioning it slightly so the contact hits a new place on the pad. This cured my issues.
Don Bitters
vk2bea