miserable upload speed

Hi, my internet service is rated for 150Mbps download and 35Mbps upload. I live in Santa Clara area.

The modem is an SB6121 DOCIS 3.0 (very old I know). Recent speed test shows download speed is around 80 to 100Mbps but upload is near 0.1 to 0.2Mbps, making internet browse a very miserable experience.

I did a speed test at 5am and was able to get an upload speed of around 5Mbps. 

Is this a sign of a failing modem? 

Will it make a difference if I buy a $100 DOCIS 3.1 Hitron CODA or a $30 DOCIS 3.0 SB6183 (both are on the list of approved device), the SB6121 is no longer supported.

Thanks,