Huuuhh...

you fire a lot of charaters by the TX-PIC, and the RX-PIC has to do his work by software.

Just give him some time to does his work.
Read the PBP-manual around the SEROUT-command and look at the DEFINE CHAR_PACING !!!

Change the filling bytes from $FF to $00, maybe the RX-PIC will find the start of a new character even better.