Just a quess since I don't have your hardware to test;

With the card operating on 3.3V and the PIC on 5V, you should change to a port with
TTL inputs. Or use some type of buffer, or a PIC that will operate on 3.3V at whatever
osc speed you need.

PortC on the 18F452 are Schmitt Trigger type inputs that need higher threshold voltages
for logic 1 than TTL inputs.