Strange heated bed behaviour

Hey Printer commanders,

I got my 1st printer running lately and I´m running into the usual trouble with calibration and nozzle blockages etc. but now I came across something I can´t explain.

I preheat my bed and nozzle and give the print order. Usually in Pronterface the temp command is displayed in the log as " T:160 @:34 B:70" until both bed and nozzle have reach the right temp, but now the bed temp just starts dropping while the print continues although the target temp is still shown as 70C

The behavior happens also in Repetier host. I checked the Slicer profiles and they are ok.

Could be lack of power from the power supply, when the bed starts moving around during printing, you'll have extra cooling of the bed.

Above the temp graph in Pronterface there is some stats showing bed and hotend temp, the @ shows duty cycle, if it's stuck at 128, you may be lacking power.

Or a bad connection.. will the bed heat up, if it's not homed?