My printer won't stop dying

I have a version of the prusa mendel i2, with an Arduino Melzi board. I run this printer through a usb cable that is connected to a computer running Repetier host on the Xubuntu operating system.

I have two problems:

1. My printer keeps randomly shutting off. In the middle of a print, the printer will stop moving, though I'm am pretty sure that the hotend and bed stay heated. Repetier initially shows that it is still printing, but after hitting the force ok button, it suddenly disconnects. If i don't hit the force ok button, it doesn't take any commands (I don't remember whether it disconnects or not). To fix this, I unplug and re plug the USB cable. I'm not sure what is going on, but I think the firmware is rebooting, since I checked the log file and I did not see any errors.

2. For some reason, Repetier host sometimes crashes. This is sometimes due to mono, but sometimes I don't know why. How do I fix this?

Are you doing other tasks on your computer when this happens? It may be suspending the program if you are not actively in it.
Is your computer going into "sleep" mode? I'm sure it's not.
Have you tried using Pronterface? It might not be your preferred choice but if it works it will be better than nothing.
Do you have the latest stable version of Repetier host? Check which version you have then check where you downloaded it. There may be a new update from your version that addresses this issue. If not, try an earlier version of the software.

Is your USB cable shielded? If not, make sure it is not crossing any power cables. This can cause interference in the signal.

Melzi has the micro SD slot. Try printing from the SD card. You can start your print in Pronterface, then disconnect your computer.

Check and make sure ALL your cables are secured properly. Loose or broken cables can cause a lost signal (especially power). Make sure you are not only attached by one thread in the wire. Also check for any corrosion on the leads.

Thank you for your answer. I have not tried pronterface, but I will do that tomorrow. The USB might be a problem as it is not shielded and is pretty long. However, even if there was interference, wouldn't that cause dropped commands, not a total disconnect?

As for the SD card slot, I using that, but I could not get it to work at all. I tried reformatting the card and changing the file extensions, but I could not get the firmware to detect the g code on the card