hi Alain,
thanks for your reply.
After the Maxim we have a high current circuit to drive the digits.

I just checked with an osciliscope to see what is happening when it has stopped working. The PIC is fine. The Maxims puts all the Segs to ground (therfore, off) and all the Digits are 5v (except, strangely Digit3)

The other thing i noticed is that when the digits starts to misbehave, i can see a lot of noise (or pulsing) in the 5v that goes to both the Pic and the maxim. I have just put a diode in to protect the maxim's 5volt supply and for the moment this seems to have solved the problem. i will post again if this hasn't worked.

best,
helen