1. The parts are all supplied from Jameco, which, to the best of my knowledge, is reputable.
2. After the write operation, the program waits 10ms before continuing, which should be well after there would be a problem with reading.
3. The supply voltage is remaining at around 4.85V, which is within the 10% tolerance of 5V of the AT28C256.
4. I just did the write operations to disable SDP, with no luck. The part, as far as I know, is new.