Found out that using an HCT to OR the /CAS and /RAS lines for the SRAM module leads to instability on this C64. They are gone with an ALS/LS or F gate.
The same was happening with the other SRAM module I used, found on GitHub, designed for the C64 shortboards. In that one I replaced it with an F gate.
Looking at the schematics (https://github.com/bwack/C64-250407-Replica-KiCad/blob/main/250407_.pdf) and saw C203 in my board is not fitted, I tried with a random 330pF cap directly soldered on the adapter between /CAS and GND. It is now rock solid. I'll do another revision with a proper place for the cap.
Also, seems I am not the first with the issue: https://www.lemon64.com/forum/viewtopic.php?t=80031
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