I have seen exactly the same problem a couple of years ago, when an interface I built was connected to some weird Japanese VME-bus computer. I used a P89C51RC+ at that time. The ONLY solution I found in the end was to configure the VME-thing-serial-port to 2 stop-bits, thus increasing the time between two consecutive bytes. Never managed to get my finger behind it, though ... and I've never ever had a similar problem again since !