Robin - if you read my post above #18 you'll see that the underlying protocol is more deterministic than this and I think the mistake is to focus only on this 0x80 symbol whereas the true info on how the buffer is structured, where is the data, where is the PID etc is actually in the bytes before being ignored if what is being at play is actually the "Connection Oriented Transport Service"
An approach would be to write an auto sync program that would scan and buffer data until it finds something that seems to make sense, that is respecting the protocol from the first byte with MSb high (since MID all start at 0x80) to a checksum and everything in the middle according to spec. That would mean possibly loosing one or two frames of data but then you know you are in sync.