Mmm! Most interesting! Good bit of detective work Mark (fancy coming to work here?)

Looking at the datasheet, that would appear to be the right thing to do (though freakishly, their example circuit shows the pin tied low which looks rather counterproductive)... definitely a better option than swapping the IC, though it's rather strange that units appear to degrade over time.

It doesn't look like this should affect the IDE bus though, which is one of the other symptoms that people see when the 4231 is playing up. Anyone got a unit that's unwell and both the 4231 and the IDE drives don't respond that they can try this on?

Hugo