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I had a quite similar problem: it improved a lot by shielding (for the porpose of my project it was enough; it didn't solve it completely, but in a 90%).
You can also try to place a small cap (~0.1 uF) in parallel in the input to the arduino. (to ground)
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Thanks. I only have 10uF capacitors, will that work as well?
Do I have to set it between digital (5V) and and GND? Without any resistor?
Or do you mean from INT to GND or the power supply?
10 uF means electroytic probably: no, try a small one -ceramic-. The electrolytic ones have a big -spurious- inductance that make them not suitable for noise supression.
If capacitance is small, the intrinsic resistance of the whole circuit acts as a resistor to make RC the filter with the capacitor you add.
Place it in between the arduino pin (interrupt) and ground the pisically closest to the chip (Atmel) you can (usually the GND terminal does).
Regards
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