No, it is not the same character always being changed. But when one is changed it is always to the next character in the ASCII table.
For example:
Should be: "Set pH in CaRx"
Displayed: "Set qH in CaRx"
If I changed it to: "Set PH in CaRx"
It displayed: "Set QH in CaRx"
I reworded it to: "Set CaRx pH" and it worked fine.
As I said it only started misbehaving when the sketch got within 1K of max. That is the only problem I've noticed as everything else seems to be working fine. This is a controller for a Saltwater Reef Aquarium.