where are you ordering parts from? it's possible the flash doesn't match because it actually isn't the part the printing on top says it is. there ARE counterfeit parts in the marketplace. what happens is a company is set up where scrap CPU boards are stripped of their chips, then the part numbers are shaved off the chips and replaced by new part numbers (whatever is popular or in short supply at the time) and resold through unscrupulous parts brokers at bargain prices. that's one possibility that would explain both of the failures you describe. do the failure on an address, is it at particular points (16, 32, 64, 96 Meg boundaries) or random addresses (as if they have been used before)?
back a few years there was an audio amplifier manufacturer that ended up with over 15,000 output transistors that turned out to be counterfeited from parts with a lower current rating.
there are a couple of possible ways of identifying faked parts. first, the ink used by real chip fabs does not dissolve in acetone. second, if there are scrape marks, uneven or curved top surfaces caused by grinding or sandpapering, curved corners where other parts have sharp corners, any of these could be the result of removing the original part number. sometimes they "blacktop" the top of the chip with black paint to obscure the original part number (again acetone can be useful in identifying this).
this may or may not be the reason for the chips failing, but with the failures you describe it is a possibility.