Heatbed shuts down - sometimes

I've just completed the construction of a Mendelmax from a kit. I've got just about everything working now except for one thing. If I try to heat the heatbed up to 110 C it shuts off before ever reaching that temperature (the LED on the board goes out and there is no voltage present at the D8 terminal). All the other electronics remain powered (e.g. there is still power getting to all the stepper motors and the nozzle.) I have managed to get the heatbed hot enough to run several prints using PLA, and I've also managed to get close to ABS temperature by incrementing the set temperature one degree at a time, but if I set it at 110 C and leave it alone, it stops heating when it hits about 70 C. If I wait a couple of seconds and drop the set temperature 10 or 20 degrees, it will usually come back on, but it never makes it all the way to 110 C without shutting off.

I have a 30 amp OEM power supply that came with the kit and a RAMPS 1.4.

Any thoughts as to what is going wrong? Is it my power supply? MOSFET on the RAMPS board?

Any suggestions would be greatly appreciated.

Is it a pid controller or bang-bang. Pid usually undershoots if parameters arent ok. For bed always use bang-bang - pid is no-no for bed.

Edit: and if it shuts "sometimes" and the mosfet is very hot when that happens then its probably in thermal runaway. In thermal protection the gate line would still be high, tho i understand from you when it shuts down its coz the gate is low, and if its low then mosfet may work correctly and the bug may be on the controller side.

Edited 1 time(s). Last edit at 07/27/2013 07:16AM by NoobMan.

It was set to PID, but switching it to bang-bang didn't fix the problem. It turned out to be the MOSFET, which was getting quite hot. I replaced the RAMPS and it is working now.

Thanks for your help.