Take the shell off... put a towel over the PCB to let it heat soak. See if it fails when hot. Cool down the RAM with some canned air turned upside down and see if the errors disappear suddenly when chips are cold. If so, cool one at a time to see which one is the temperature sensitive one.
Unfortunately, I'm in the states and it's insane shipping stuff these days. Ping Caius - he's in Italy, but not sure if he's on these forums. I know he's on KLOV and the Arcade Projects forums.