<p>I have a prusa i3 with Mendel firmware and a RAMPS board.  Recently it has been randomly stopping during prints.  The LCD screen will lock up, the print will stop, and the heating elements will turn off.  Pressing the reset button on the RAMPS restarts the system and it works fine.</p>

<p>In addition to stopping during prints, it has also frozen up while just sitting while on.</p>

<p>My first thought is the power supply (12V 30A) is going bad, but is there anything else I should check before I buy a new one and replace it?</p>

<p>Update:</p>

<p>I replaced the power supply with a new one, and the printer did not stop and completed a print.  I am voting to close the question.</p>

It could be several things.
Your ramps board is overheating or has to much load on it. If you're not cooling the ramps board adding a fan may help the issue.
I know Robo3D had this issue and started shipping with a fan to cool the ramps board.
The ramps/arduino board could be faulty, the firmware may have gotten corrupted or the current version has a bug in the code.
If you are not printing from the sdcard on the lcd controller and using software through a usb connection, that computer may be causing the issue as well.