At first glance -

Can't see RETFIE anywhere - snipped or not there ?

Config not included - is WDT disabled ?

=============================================

btw I've just finished a similar system but being short of pins
(using F628 and two small Scenix) data was transferred via
a 74HC164 and flow control effected with two flip-flops. The
flip-flops act as a timing buffer between the slow PIC and very
fast Scenix. Data is sent, received and acknowledged as and
when each micro has time and the others know what state the
various transfers are in by polling the flip-flop outputs

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