Serial port bizzareness

Hello everyone,
I hope some of you guys could help me out? I've got a CNC milling
machine hooked up to an old (Pentium 166) PC running Win98 via the
9-pin serial port (com1). It works fine, but no matter what settings
I change, I can't get it to work on com2, and more importantly, when I
try to get it to run on a brand new PC (which it will have to do, as
we want to get rid of the old one) it doen't work properly.
On the new PC, it only seems to want to RECEIVE data from the
machine, but when I try to TRANSMIT data to the machine, it does
nothing.
When I tried cables with the Rx and Tx swapped around, it didn't
even want to receive data. I've tried it on two other new PC's runing
XP with the same results. There are no conflicts and no serial
mice/modems etc.

Any help you guys can offer would be such a great help!

Thanks,
-Tone.

I am sure you know all this, but I'll go over it just in case:

To a motherboard, there is no difference between Com 1 and Com 2. You
can even switch them around in the CMOS settings, assuming your Com
ports are built into the motherboard. Now, perhaps something is
sharing the com port 2. If you have a device on Com port 4 (like a
modem), then it is sharing Com port 2, and that is very bad. Com 1 &
3 devices are shared, Com 2 & 4 devices are shared. You can look in
Windows 98 device manager to see what ports show up there. You should
see Com 1 and Com 2, and no other Com ports. If you see others, then
there is your problem. You can also see that same information when
you boot your computer and watch the device listings go by on the
blackscreen.
If no other com port is there, then you know that probably nothing is
sharing the port. That's good. So then one thing you can try is to
set Com Port 2 in the CMOS to 3F8 IRQ 4, and set Com Port 1 to 3E8
IRQ 3. Then see what happens. (Normally default setting for com port
1 is 3F8 IRQ 4 and com port 2 is 3E8 IRQ 3.)

The other thing is that some motherboards had a wee little ribbon
cable to com port 2. If so, it could be defective. You could have
corrosion on the Com 2 pins since it is used to little.

I wonder what speed the Com port is. Some very old computers had the
slower com ports, but I am pretty sure that anything over 40Mhz had
the faster ports. Just a stray thought...ignore it.

That's all I can think of for now.