Steppers + PWM + Powersupply = Dead Arduino

I've built a standalone Arduino that has a sparkfun easy driver attached. The Arduino turns a motor via a mosfet that runs about 15k RPMS and pulls 25 amps at 12v. The stepper motor moves a chain right before this motor turns on. The problem that I have run into, is that I used a step up DC-DC power supply to up the volts from 12V to 24V to get all the torque out of the stepper motor. This works great (lots of torque). I've got 24v to the motor input on the easydriver and 12v on the arduino but when the step up power supply is on and the 25 amp motor fires up, the Arduino dies. I have to kill the power to the Arduino to reset it. Everything is powered by one 12v battery.

My theory is that the DC-DC power supply is oscillating and combined with the noise of the motor, it messes with the oscillator on the standalone arduino. I do have supression diodes on the 12v motor. Has anyone else run into this situation? Any ideas on a fix?

Have you applied Grumpy Mike's suggestions...

If you are trying to pull 25A from a battery that isn’t sized for it, there is a good chance that the problem is not decoupling but just voltage drop. What is the voltage at Vin on the Arduino? If it falls below about 6.25v, then the regulator that produces 5v for the ATMega chip won’t be able to provide enough voltage any more.

With a step-up converter, if you are delivering 25A at 24v, then you may be asking for 60A out of your 12v battery.

The step up power supply is 12v 2A in and 24v 1A out. The only thing it is hooked to is the motor supply on the stepper driver. The stepper is a 40 Ohm, 750mA motor, so it needs 24v to get 100% torque. Everything else in the circuit runs off 12v. The Arduino has a 7805 regulator.

The battery is a lead acid 25AH battery. I am using an H-Brdige motor circuit to power the 15k rpm motor, and I'm using PWM from the Arduino to ramp up the speed over about a 2 second period. The problem is when the step up power supply and 15k motor both have power turned on. The 15k motor runs for about 1/2 a second then the Arduino dies. I put a relay on the PS so that it only has power when the stepper motor needs to run. This has solved the problem because neither the power supply or motor are turned on both at the same time. However, it seems like a lack luster way to solve the problem. It sounds more like Grumpy Mikes decoupling needs to be applied.

Turns out the Arduino was decoupling. I added a .1uF cap acros the + and GND right next to the ATMEGA chip and after a few trial runs, it appears to be working now.

Thanks for the help!