I've had uv eproms that got flakey. Reading them at reduced temperature, to a file and repeating a higher temperature should ALWAYS read the same.
Surprise surprise!! Not always. Usually read better when cooled if they are getting near the end of life. The charges on the bits, internally stored on capacitors slowly leak away, or possibly get zapped by cosmic rays, (who really knows).
After 10 years I have found plenty of unreliable ones in all sorts of equipment.
There were also specials made -- apple computers did it -- crude copy protection -- that had a swapped chip select line for an address line. Programmer would read them supposed OK (but only half read really) then you programmed a normal chip. Checksum matched but they didn't work. (wtf !!)
Buy new ones and reprogram.
I use my UV eraser for curing epoxy these days. hahaha
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.