The problem is not every AES outputs clean sync. Using a sync stripper cable won’t cure the problem. There are cables wired for clean sync from the AES, you could try asking Rob at Retro Gaming cables. If the cable still doesn’t work you might need a csync restore mod doing.
I’m not 100% it is just due to the cable either, it maybe you need a more sophisticated RGB bypass doing on the machine, but don’t quote me.
PSU is unlikely to be the culprit. Typically PSU noise manifests itself as moving/wavy interference patterns.