But usually when a capacitor is shorted/leaky, the DC voltage is lower than it should be.

If a capacitor is open I will get loads of ripple on the rail, but I've never seen an electrolytic capacitor go open.

Also, I noticed that the Boards in the 128K/512K don't have as many problems as the ones inside the Plus, they are the same exact board (as far as I can tell) but with older components. Maybe Apple decided to use lower quality parts for mass producing the Plus.