I've made a PCB of my own design using the FTDI FT232R chip in basically the same configuration as the Arduino Diecimila.
In noisy environments (e.g. motors and drills plugged into the same power board) the chip seems to "lock-up"/crash, and the only way to get the comms back is to turn the power of and then back on.
Is this a common problem with the FTDI chip or Arduinos?
Hopefully I will get a chance to test my genuine Arduino board tomorrow and see if the problem is limited to my design.
Missing or too few decoupling capacitors in your power supply traces. You need to filter out the noise. This is a common problem in logic circuits. Usually where motors are involved, it's a good idea to have separate supplies, one for the motor, one for the microprocessor and electronics.
InterSil Application Notes on "Choosing and Using Bypass Capacitors" http://www.intersil.com/data/an/an1325.pdf
I had (as far as I could see) plenty of caps. When I first noticed the problem, I went and stuck extra caps all over the board (basically wherever I could, both close and far from the FTDI chip), but to no avail. I tried different types (ceramic, electrolytic, tantalum, combinations) and values, but the problem persisted.
I don't have a ferrite bead, which is about the only component I can see that would be affecting it.
The problem seems to lessen when powering directly from USB, but it completely vanishes when I plug in the AVRISP II programmer. :-?