Some ideas still:
Because of the "finger on the plastic working OK effect" there must be a high-impedance or undefined stuation somewhere.
Does the NPN-transistor get enough input from the external to pull the interrupt pin low (enough)? What value does the pull-up have. If the value is too large you might have a capacitance problem.
Also It could be that the NPN is not configured as it should be.
Does it have an input seriesresistor and a resistor from base to ground? If not the NPN could get saturated and behave strange.
Then with introduction of a noise (from your finger) the input "tripps" into the desired state.

I made a schematic for you:
http://members.tripodnet.nl/aadplagia...xt_int.jpg

mail me at: aad.sieraad@quicknet.nl if you need to.