Hi Ralph,

I know the 18F series are pretty suceptible to noise in general, so that may be what's happening. Are you using 0.1uF caps from the PIC Vcc & gnd pins with the cap located as close as possible to the pins? Is your power supply clean & stable?

A while back several other folks mentioned similar problems when migrating from 16F parts to 18F parts. I have never had this problem with any 18F parts, but you're definitely not alone.