A - If you will me using the on board antenna then you should not be placing conductors (ground) underneath it.
Will use the external antenna.
B, C - You have a lot of vias connecting islands of copper that go nowhere.
D - All the current that is going into the ESP8266 is going through the one lone via at D.
I will take it into account on future revision! But isn't it shielding? Both areas are well connected, could this help to reduce EMI?
What voltage regulator are you using? And what is the input voltage?
I am currently using the standard ams1117 3.3V power supply. This could be a point, but it's working quite reliable on other boards, since the esp is a digital IC. So I already applied clean 3.3V bench supply but this didn't improve the situation.
Anyway great job rudy! My appreciation!