Have CM400 modem with upload slowdowns that happen at regular intervals

I have a CM400 modem and lately I have experienced a slowdown of only my upload speed starting at a specific time of day between 7:30pm and 8pm for the past week. My upload speed goes from 30mbps before that time to between an unusable 6 to 9 mbps after that the slowdown ends between 4 and 10am the next day (I wouldn't know I'm asleep then). Only to repeat the next day. My download speed remains the same acceptable normal speed its always been at all times of day. This has happened every day for the past week. I have directly connected my computer to the modem bypassing the router and the problem persists. I have contacted my ISP and they have said that my connectivity is fine and ofc they say my modem is to blame for the issue. They sent a repairman out. In addition to hard selling camcasts rental modem He helped replace some badly worn cable leading from the telephone pole to the house and I was hoping that would do it. but the following evening the problem returned and wil;l most liekly return tonight. I write to ask what could be causing the issue? Its an old router thats been serving me loyally for 7 years so maybe its time for it to go to the great modem recycler in the sky but I just wanted to ask here if they had every heard of such symptoms and if there had been a fix for it. Firmware problems? A virus that affects the modem only? some electronic noise that cuts in at only evening? If the modem speed was failing at random times I would just chalk it up to age and get a new one but the symptoms on their face look super suspicious. Thanks for any attn I get here and I can answer any questions.