Why does one of the two dimmer start flickering when I put them together?

Recently, I have come up with a problem I cannot find an answer to. I hope you can point me in the right direction.

My problem is as follows: I bought two diy dimmer sets for 230VAC, max. 100W (like the one in the picture). Because the Halogen light I want to control is 100W I replaced the triac that came with the package with a BT139 (just to be on the safe side). After putting them both together I noticed that one of the two starts flickering below, say, 50%. I have tried switching the bulbs, switching the wiring order, I measured the potmeters resistance (which is fine), I measured the voltage drop across MT1 and gate (0.412V, which is about the same ass the other triacs), all soldering and wire connections are fine (<1 Ohm). Unfortunately, that is about as far as my knowledge goes.

For understanding the circuit, I found a scheme of exact this dimmer(the Diac is a 30V one, the triac is a BT139: http://www.kynix.com/Detail/1255347/BT139.html)



Can you guys give me some hints of what I can do?

Thank you!

P.S. I only have a multimeter, I don't have a oscilloscope at my disposal...