So, I was powering everything from arduino uno board.
Today I repurposed an old Sat Receiver's power supply, which supplies multiple voltages (3.3, 5, 7 and 12V, all with common groud), and powered everything (ethernet shield 3.3v, DSM501a 5v, dht11 5v and the arduino itself with 7v.
still to add a gas sensor, as soon as it arrives at my location.
The insane behaviour seems to have stopped.
Although, I had one 20min pause, in about 5 hours of sending data every minute. Not perfect, but can't complain. It could be internet connection, or even a thingspeak problem.
Can't debug live, because the device is far away from home.
Will be now for a 48h run to check for stability.
BTW, channel here: https://thingspeak.com/channels/230655 