As previously noted, you can tie your interrupt pin to a high level state and make sure that irregularity does not follow. Make sure the CPU is not constantly being reset via the /RST pin. Make sure your clock source is stable. Make sure you do not have multiple interrupts going on at the same time and this one has a low priority. The interrupt pin should have an internal pull-up resistor (no worries there). Another tid bit is that if you trigger in the nanosecond regime your CPU may not be fast enough to poll the interrupt port to cause an interrupt. That is the CPU side, more likely you might find your comparator to be at fault because you stated that even when the IR portion is not active it still triggers. Which means your comparator is not working as you intended.