Sounds like the 5V regulator on the Arduino is overheating, which is a common problem with Arduino/RAMPS electronics when you start powering other things off the 5V rail. Workarounds include:
1. Supply external 5V power to the Arduino.
2. Rewire the LCD backlight so that it takes power from +12V through a series resistor instead of from +5V.
3. Replace the Arduino by a Taurino, which has a higher-rated 5V regulator.
4. Replace the Arduino/RAMPS by more modern electronics with fewer limitations.
Edited 2 time(s). Last edit at 08/15/2015 06:32AM by dc42.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].