Multiple chip failures are fairly common. What kills one is likely to kill another from the same batch, ie. they are likely to have the same bad spot on the die as the mask is stepped across the wafer. As for dying in storage, it's common for the analog section to drift as aluminum capacitors lose their oxide dielectric when not carrying a charge. They need time under bias to reform the oxide and for leakage to decrease. That's why old tube radios are usually gradually awakened on a variac. Anyhow, it's pretty likely that +5V is a bit lower than it used to be, and those 2 chips need a full 5 Volts to work. Like an overclocker raising his RAM voltage to run his memory faster than it's rated speed. I can think of a couple more similar explanations, but I have noticed the same effect, and the Apple-branded RAM failed fairly commonly when it was new. Most of it was made by Micron, and their early chips were flaky. Flaky chips are good when they are potato chips, but not the silicon variety. (Micron was founded by an Idaho potato millionaire.)