Even on the 'good' modem I occasionally get drops in the RU connection (ie. I get signed out and also lose remote connection). On the good modem this doesn't affect anything else; I don't lose the ability to navigate.
What I am currently thinking is something like this: perhaps my internet is not very stable. Let's say I get microbreaks in the signal. For some reason, this computer deals with them OK when connected via some modems, and with others it gets confused/blocked somehow.
Might this make sense?