heated bed doesn't keep temperature, regardless of setting

Hi,
my heated bed can't keep temperature. When I try to set a temperature, the bed will heat for a considerable amount of time, and then drop down to room temperature and refuse to heat anymore until the printer was turned off and the plate cooled down.
By exambple with a set temperature of 75°C the temperature rises to 61°C, and then drops again to 19°C. It is not always 61°C. sometimes its less, sometimes it keeps temperature for a while and then drops.
I heated the printer to 60°C, then did a PID test on the bed, the results were
Kp: 462.43
Ki: 90.46
Kd: 590.98
I entered them to Marlin, uploaded it again, but nothing changed.

Any Ideas?

Is this a RAMPS? if so I would suspect your polyfuses.

Yes, it's a RAMPS 1.4, sorry for not mentioning it earlier. Which part of the ramps are the polyfuses?

The orange or yellow rectangles.

Thanks for the explanation. Indeed, I removed all the electronics from under the bed of the Kossel to a place next to it, and placed a fan onto it, and now it works. Thanks mate!