PICJUG: A FOSS Replacement for the FT232 USB Chip

"The PIC works from 5V down to 1.8V..."

You might want to double-check the datasheet (and double-check with Microchip support as I've found bugs in their datasheets).

I looked closely at the PIC16(L)F1455 and PIC16(L)F1459 from the same family, and those chips have separate SKUs for 3.3V-ish and 5V-ish. For example, the PIC16LF1455 can operate at 3.3V, but you need the PIC16F1455 to operate at 5V; i.e. the "L" in the part name is significant, signifying lower voltage.

Even if the L part works at 5V, you may be running it out of spec.