Hi Malc_c

I was curious to see this thread and got into this pretty late.

How big is the serial data packet that you send down to the PIC?
From the details you gave, I think you have problems PC->PIC (downstream)
If that is true, are you buffering your received characters on the PIC? If not, then the least you can do is make the PC space out the characters in time. Put a few mS gap between each character sent to the PIC to avoid losing characters on the PIC
How do you synch the receive to the start of the incoming stream of data? Do you have a synch character to look for?
It will be better to look over the whole code to find the problem.

Regards