Voyons voir... mmm.

As i already tested, CTS and other flow pin may react slower than you want. So may i suggest a special home brew PC software that you'll have to build?

In fact, when i need to dump XYZ data, i build my VB program, The PC send XYZ function and sit for a PIC reply. The PIC sit for the PC data, the PC send the data finishing by a Checksum (or equivalent). When the serial timeout occur, the PIC do the checksum and TADA.

Well, really rough explanation but it's suppose to be enough.

Bonne chance!