Looks like it is the USB connection. I printed 2 prints with the SD card that were 45 minutes & 30 minutes long. I tried a 2 hour print from proterface again to see if taking all the parts off & back on again had fixed the problem. It failed after 5 minutes. I tried that same print from the SD card & still had the USB connected to my computer & pronterface was still connected. The print did fine, but proterface had lost its connection at some point, so that seems to point to the problem. I might take the boards apart to see if I can see some loose solder joints to fix before replacing the board. Does anyone have a suggestion for the best source for a replacement arduino mega 2560? Looks like makerfarm has good price of $20 for this & probably more reliable than some of the reviews I have seen on Amazon.