Have you tried using SERIN2 or DEBUGIN..?

See what happens with something like this:

DEFINE DEBUGIN_REG PORTC
DEFINE DEBUGIN_BIT 7 ' Use RC7
DEFINE DEBUGIN_BAUD 9600
DEFINE DEBUGIN_MODE 1 ' Inverted

DEBUGIN 20,fail,[WAIT("ZOM"),z,zc,f,fc]

I have found (on several occasions) that just using a different serial input type/command does the trick.