I think it's a bug in the TI-OS that writes invalid groups at some point. I don't know what causes this, but I've run into problems with it at times. Makes me want to write a group file recovery tool for this type of problem
Other than that, maybe you have a piece of Flash that's wearing out. There's a strip of code in the bootstrap that performs this test on Flash, but it wipes everything out on the calc, including the OS. You can jump to this address, as it usually starts with "LD HL,<somenumber>" where that number is a pointer to the text "Testing Flash"
I'd definitely want to try the ON+CLEAR thing first, as that seems to have helped a lot of people out on those that asked about it on UnitedTI. Something to the effect of preventing the VAT from being reconstructed or something so you can go straight to clearing out whatever from your memory.
After that, get an OS that works. I would grab one from another person since the newest one is like total crap.