The board itself is very reliable, but occasionally the serial port hangs and needs to be disconnected and then reconnected - though this might not be the Arduino's fault. I have, however, seen this behavior when using it with both Linux and Windows boxes. But I'm connecting through a cheap Chinese 150 foot USB cable that I know to be unreliable in electrically noisy environments.
Is this the arduino serial board? Like many designs that try to stay simple and inexpensive, it plays the game of basically stealing voltage from one pin to get the RS232 voltage for the other. I've had problems with other devices that do this in the past, although I haven't used the arduino serial specifically.
A true RS232 tranceiver (MAX232, MAX233, etc) may help solve these problems.
If it's really noisy you could also try to use RS422, which IIRC uses differential signalling. Either use a serial port that's RS422 capable or convert from 422 to 232 at the computer end.
-j