It sounds as if when you command X+10, the printer is interpreting this as a command to move to absolute position X=10 instead of to X=10 relative to the current position. In other words, it is ignoring the G91 command to switch to relative mode that Pronterface is presumably sending it. Perhaps you broke something in Marlin when you modified it. Try sending G1, G91 and G90 commands to pin down the problem - see [reprap.org] for descriptions of these commands.
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