🚀 go-pugleaf

I have 2 onboard 16550 serial ports that have been working great. I need
to install a third port so I got a basic 8250 seial card and installed this
card as COM1 I then moved the 2 onboard serial ports to COM2 and COM3.
I know all the hardware is working correctly by themselves.
I have connected:

ttyS0 Testing (COM1 8250 serial card IRQ 4 at 3F8)
ttyS1 Modem   (COM2 16550 onboard serial IRQ 3 at 2F8)
ttyS2 Mouse   (COM3 16550 onboard serial IRQ 4 at 3E8)

When I boot all 3 are recognised by the BIOS and then by the linux probe.
I can then use the modem and send data out the testing port. Both ttyS0
and ttyS1 work correctly.
It only when I run X that my problem shows up. Under X, the mouse works
and the modem works.
When sending data out the testing port (ttyS0) it is VERY slow but not
corrupt. A single character is sent about every 10seconds. Too my
amazement when I moved the mouse the data then got sent. It seems that
when the mouse (ttyS2 Irq 4) generates an interupt the service routine
also checks the Transmit buffer of ttyS0 also at Irq 4.
Reading the Serial Howto it seems I should be able to share interupts
like this so what else have I missed. 
Even though the Linux probe finds the seial ports correctly I still
use setserial to make sure the ports are configured correctly.