You mention you have tried different chips and different programmers, and still have ever changing results? Had those borrowed chips been heavily reprogrammed, too? This sounds very unlikely.

But what is the common in all those combinations? You? Maybe you have some magic power influencing programmable memories :-) Seriously, don't you wear something that causes you being charged e.g. when walking, and then discharged in the vicinity of the chips? Or isn't there anything close to the place you keep the programmer(s) that emits strong electromagnetic "smog"? Or maybe the mains power carries some strong interference? Something sparking - motors, contactors - working one day and resting another?

A story: Coincidentally, on Friday I was shown an AT89C52 by one of our serviceman, which could be erased completely, but som of the bits then refused to be programmed. We tried to program it to all zero, and a couple of bytes remained 0Fh, some 0ch, some 04h...
The circuit which contained the chip had been zapped by a piezolighter...

Jan Waclawek