client.loop() should return after ONE message received

[Suxsem]

I have this problem: I have a lot (about 10) topics that have retained messages.
After connecting to the broker I subscribe to all of them in one call to client.subscribe.
I start to receive the retained messages but after some of them the esp8266 freezes.

I put some debug instructions inside the callback function and before and after client.loop():
Serial.println("before");
client.loop();
Serial.println("after");
delay(500);

This is what i get:
before
new message
new message
new message

and then the esp8266 freezes. the delay is never called.
So, why client.loop() processes more then one message? How can i slow it down to prevent esp8266 from freezing when it receives a lot of messages at the same time?

I'm using QOS 2 for all pub/sub operations.

[Suxsem]

p.s. when the esp8266 freezes I still have a lot of free ram (more then 40k).
in mosquitto I got "Socket error on client ***, disconnecting" right when it freezes (mosquitto doesn't seems to need the missing keepalive packet to understand that the esp8266 freezed)