Who made your 65C02? Was it WDC?
If it's WDC, then there are a few pins that should be held high with a 3.3k pull-up resistor. Such as the
BE pin which is only on WDC versions.
BE (Bus Enable) should be pulled up.
Some example taken from http://wilsonminesco.com/6502primer/MysteryPins.html
VPB (or VP):
With WDC's 65c02, leave it unconnected. With anyone else's, ground it.
...pull this up to +5V through a 3.3K resistor.
You'll only find this pin on WDC's 65c02's anyway, not on those of other manufacturers.
Point is, 65C02 isn't always just a drop-in replacement. You need to make sure all of the pins are grounded, pulled-up (or down), etc. I don't have the A2 schematics in front of me...but make sure that all of the pins are taken care of even if there isn't a card in the system.
Also, this is another good reference on IRQ/NMI of the 65C02. I highly recommend reading the entire "primer".