How does changing arduino power supply from USB to Adapter change the circuit? I have a circuit running with capacitive Sensors, that also has an LED strip. everything works fine when using USB power from a laptop (connected to the mains) and 12v 2A adapter powering 1m LED strip. 

When the power supply is split and the arduino is also powered from this supply ( using adapter port ) the capacitive sensor begins to give junk values. 

What changes in a circuit that would create such a porblem? Could it be a grounding issue? How else to ground this arduino to behave as it would with a usb connection?
I have run into problems like this before, often it can actually be that the voltage is not steady due to the LEDs sourcing current from it.

You may be able to solve the problem by adding a capacitor inline with the power input (from the +12v to GND). I would probably start with a value of around 10 &mu;F and increase it if the interference continues. 

Capacitive sensors and very sensitive to voltage fluctuations, you may even want to put a second capacitor across the VCC and Gnd of the sensor.