I don't know if you can connect the midisport
thru the serial port, as probably it's the USB connection.
Otherwise look for updated drivers just in case there are, as USB sucks for midi timing!

Don't forget , just in case you did not do this, to disable midi echo in cakewalk.
I am not sure about CPA 6 as I did not use that version a long time, but I think there's also a midi thru option you can turn off!

I also use a PIII 500 and have no midi timing
problems at all, and my cables are much longer!
I do use though the serial connection of my
midi patchbay. (AMT8)