Your modem is fine. The problem is the upstream power level. Something is wrong woth your cable provider's plant. I fought with mine for years. Nothing happened until the device in question deteriorated to the point that it screwed up TV as well as internet. Then they fixed it on a Sunday night.
You might try an RMS RDA-L1R drop amp. Nearly all drop amps are one way. This is one of the rare ones to boost in both directions. It will give you a 10 Db boost upstream, reducing the power level into the 40s.
It worked for me.