Thanks to all for the suggestions-
1/ I am not using a hub.
2/ OSX 10.49 is the version of OS
3/ I have reinstalled the drivers(2.2.7)
The port seems to have stopped disappearing, BUT I did a comparative test on the G4 machine(only have this for 1 more day)
On the G4:
Upload a simple sketch to read a pot. OK
Set the rate quite high 38400 baud. OK
run the terminal program Zterm to read th data , which is a value between 1 and 10. OK
run Arduino 0008 and use the serial monitor. OK
This works perfectly. i.e you see:
etc... etc.. as you turn the pot, with a steady stream of numbers.
EXACTLY the same test on the macbook:
The data comes through in bursts, with errors too, and looks like this:
note the gaps and how some data is 'doubled up'
Running Arduino 0008 and using the serial port monitor shows the same result.
Note: setting the baud rate to 9600 does NOT cure the problem.
I can only think that the FTDI driver is buggy( I emailed them , but no reply yet), or something is interfering with it.(??)
The FTDI driver makes each usb socket on the computer appears as 0000101D (rear USB socket) or 0000103D (front socket)
I've used these USB sockets for other things without any problems. The whole computer works perfectly in all other respects and is very very reliable and stable.
I may try making a new user account and running it here to see if that makes any difference.