I checked everything; wiring, polarity, changed the capacitor, changed the led, changed the power supply, even change the chip! The only thing I can't change is the value of the resistors. All I have are 1k (which I use as R3) and 500k potentiometer (which I use for R2). Obviously, when I turn the potentiometer down, I get a very fast flashing rate and the "double flash effect" is not perceptible. At this point, I think the potentiometer might be the problem... not that one potentiometer (I've 50 of them and they all react the same way) but rather the use of that type of potentiometer. I'll do some more tests when I can get some new resistors.

Meanwhile, let me know if you think of something.

As always, thanks!

Cheers