I think the problem might be on the TX side ...
Code:
CNF=0 
serin SFVC,4,100,SR,[CNF] 'RECEIVE CONFIRMATION.
The CNF is in the "qualifier" brackets, so SERIN will wait to receive a 0 byte.
But 250 is what's being sent for confirmation, and it'll never see it.

To receive the byte into the CNF variable, it should be without the brackets.
Code:
CNF=0 
serin SFVC,4,100,SR,CNF 'RECEIVE CONFIRMATION.
Then the

IF CNF <> 250 THEN GOTO SR

should work.