It's probably the most intresting idea so far. Today I tried banging the box as hard as I could but it still works flawlessly. I've not been able to provoke the behaviour one single time. The intresting thing is that it seems to do this for a certain amount of time and then pops away like if there were some capacitor involved in this event. Since this event occurs so rarely it is probably nothing to worry about in practice but being a perfectionist I really don't like faults like this. The most important thing is that there should be no flaw in my design so it must be some poor solder joint or something like that.