The "Unicast DSID PSN Startup Error" means that your cable modem/gateway encountered some type of line issue, usually with the upstream signal, and is not able to properly connect. It could be the SNR caused by ingress leaking into the coaxial line, a bad connection on a coax faceplate, variation in current, frequency, etc. causing the cable modem to reboot.
First thing to look at would be reducing the number of splitters on the line, checking connectors, replacing attenuators/amplifiers, etc.
If there are other problems with signal levels, it makes sense to troubleshoot them first, before tackling this error, as it is only relevant in the context of other errors around it, how often it happens, and how the signal levels change when it happens.
Also see:
What cable modem signal levels are considered good?DOCSIS T1, T2, T3, T4 Timeout Errors?
