i have this problem for months...i made a coin counter ..and it charges pulses 0n interrupts .because of emi..or something.. when the 220 outlet is disturb..so i add capacitor ,i tried regulator,i shortened wire s from coin acceptor to 328 int pins,pull-up resistor and it didnt fix the problem....opto coupler didnt help either...help pls...
Have you tried posting a rant on the Arduino forum without even a schematic or some code for the community to review?
Oh, I see that you have.
The following may help any analysis by a third party: 1. Schematic 2. Code 3. Link to your "coin counter" sales/support page
If the coin counter itself is very susceptible to mains (220) interference, you may have to solve that separately.
(echoing what Bulldog wrote as I was typing this)