> I have tried to connect my development equipment
> like this........

> PICSTART PLUS to COM3 (Addr 3E8 Irq 4) using MPLAB software.
> Serial I/O of my PIC to COM4 (Addr 2E8 Irq 3) via Windows Terminal.

> Presumably this is because of the interrupt conflicts.

You are right, you are sharing IRQ3,4. Try getting a serial board (Siig,
or equ.) that allows you to map IRQ9 or another. I've seen them for $25 or
so.

-Tom