All you have is 2 internet customers in this bldg? If so and they are having the same problem maybe its the node transmitter going bad, ran into a similar problem with a node feeding about 100 customers and all the modems seemed to flap at the same time and I had good upstrean lvls dnstream lvls and c/n was good.
This is CABL.com posting #243501. Tiny Link: cabl.co/mbbvB