As Humpalot says - the 5v has to be dead smooth, if you have a CRO then you can test this for ripple.
With the voltages on the PIA outputs - the 5V is a high. If you have several voltages at 2,5V then consider this your low, anything that is in the middle of these 2 readings should be considered suspect.
Just to recap a bit - do you have the 5V at pin 37 on interboard???
