Can you link the Arduino_test patch, not sure what it does. May need to change the serial communication speed of the serial port when it opens in PureData, or could be something else.

You could also implement a simple passive lowpass filter from the output of your LDR, this would attenuate the high frequency changes and, ultimately smooth the analog signal arriving in PureData. just a simple resistor-capacitor network would do the trick, check google for a calculator depending on your cut-off frequency.

Of course though, the formula is

Fc = 1 / 2pi*R*C

I realise this is an old thread, but thought i would just add this for anyone else having issues.

Cheers