Oh, hello - sorry, didn't see you there
I seldom look to see who is actually asking the questions, as 99 times out of 100 it's a new user
Right, so now I have some context, and I know you're not someone with no knowledge, so we can get on to proper troubleshooting
It's unlikely to be the power supply (I have seen your PSU arrangement in older images, and it looks fine - pretty much identical to ones I have done in the past). Mains noise is unlikely unless it's in a very very noisy area, which given the number of fans around it may be I suppose.
That being said, you can never have too much decoupling. I'd be inclined to add a capacitor directly to the logic power pins to the LCD - say 100uF or similar - to see if that helps.
Is there any chance you could get a picture of the corruption?
How have you got the LCD connected up? Are you using an I²C interface chip as we talked about once, or have you used another 328 chip? (It looks to me like you're feeding it direct from one of the existing 328 chips? which one is that?)