All of my linux machines are exhibiting poor preformance with FTDI usb/serial devices. This problem seems to affect my Laptop (Laditude D630), my desktop (Self Bulit), and even my Raspberry Pi. None of these systems work properly with either of the FT232 based devices I own: A RepRap printer with a Melzi board and a Arduino Duemilanove. The problem manifests itself in lots of strange ways. Primarily, it results in magor slowdown in some programs that recieve from the device. In pronterface, the controll program for the 3d printer, I need to hit connect multiple times. When it finaly does connect, you can see the startup info coming in character by character even at 250kbaud. With Cura, an alternate program for the printer, I had to go into the source and increase the timeout in order to connect. With the arduino, the same symptoms (slow transfer) appear with the serial monitor built into the software. Uploading sketches to the board works as fast as always and without errors, though. Even when the board is not sending anything, the software is still reading in new data in a futile attemt to keep up with the buffer. Most annoyingly, some of my processing sketches (programs) wont even run: the device buffer fills up and overflows (I believe) , causing lost packets and loosing sync. An example of a processing sketch that fails is the Razor AHRS 1.4 test sketch. In my own sketch, which also works on windows but not linux, I had to add a delays to avoid dropping packets. I believe that long ago all of theese issues didn't occur on linux, and the have never occurred on windows. I just tested the same processing sketch on my laptop under windows and it worked flawlwssly.

I have tried two different usb cables and it made no diference.

Is FTDI on linux just broken beyond repair?

I have been putting up with theese issues for months, but I is getting to the point where I will need to go back to windows if I cannot fix them.

Well, all I can say is: I've noticed the same problem and on one machine it was, sadly, solved by going back to Windows. Intermittent problems with speeds, data loss, losing connections are unfortunately very difficult to diagnose. So: (a) you're not alone (b) unless you can generate a reproducible consistent error it'll be hard to fix. If you can, then perhaps a bug report submitted to the kernel developer responsible to FTDI might help everyone.