The accelerometer seems to be working perfectly on my setup. Until i start increasing the speed of my brushed motor through a motor controller (accelerometer seems to stop working after this). I have decoupled the power sources and am still having the same issue. Both power supplies share the same gound though.could this have anything to do with this?
Both power supplies share the same gound though
That is the way you have to keep it unless you go for an opto coupled output to drive the motor.
I have decoupled the power sources
You may have some decoupling but you patently do not have enough.
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
You could also try putting a small capacitor 100nF across the motor to cut down on the noise.