Sounds like an epidemic or printer ebola. I have the problem too. I have a Makerfarm with RAMPS and Marlin. I can heat the bed and heads and it will sit there heated all day long. I replaced the power supply just to eliminate it as a cause and that didn't help.

What happens to me is that it will print for about 45 seconds and then it slows down and just halts. You can hear it slow down over about 3 seconds time. After it's stopped, if I try to raise the head with slicer controls it makes a buzzing noise in both Z-axis motors. I doubt both motors have gone bad. I'm guessing my RAMPS board has failed in some way. I've ordered a new one. An earlier post said a new board didn't solve the issue. If it solves mine I'll let you know. I too could print a 20mm cube just fine. But, anything much larger or complex would not complete.