Bigger Wall Wart?

I have just completed a standalone Polargraph ( http://www.polargraph.co.uk )

It is a Arduino Mega running two stepper motors (nema 17) a micro servo and a TFT shield that also has a micro sd reader inbuilt. It is powered by a wall wart outputting 12 v DC 1 Amp.

When the servo moves the screen cuts out. HOWEVER when I have USB power as well it's all sweet. Do I need a wall wart at 1.5 Amps. or 2 amps?

1 Amp seems kind of puny for 3 motors, is the WW a regulated type?
Have you tried with only 1 stepper or less the servo?

Servos take 6V, not 12 - so how are you powering the servo?