Terraviper-5:
I have traced the problem to the Arduino side, it is happening even if I disconnect interrupt pin from optocoupler. Internal pullup must have problems...
The output of that opto coupler is more like an analog signal than a logic signal.
You need to first find the best value of pullup resistor to make it high voltage when
off and low voltage when on, then you should pass the signal through a 74HC14
or similar schmitt-trigger gate with hysteresis so that it becomes a fast
logic signal with high noise-immunity. The built-in pull-ups are rather high values,
try something like 2k2 instead.
Logic signals are required to switch very fast. In a fraction of a microsecond.
The photo-transistor in a standard opto coupler doesn't switch fast enough to
be reliable without a schmitt-trigger afterwards.
There are opto couplers with built-in logic-conditioning circuitry that would
do the right thing, though they are a bit more expensive.