SN754410 bipolar stepper problem

I am using an SN754410 to drive a bipolar stepper using the typical 4 pin setup. On a breadboard, it works fine. On the project board, it doesn't and overheats. The stepper rattles but does not respond to switches. This photo of the very poorly designed board will hopefully illustrate the problem. The Mega is powered by a 6v dc to dc converter from an 8amp, 12v power supply. The lower chip is an ULN2003 driving a unipolar stepper without a problem. I am hoping the photo will suffice. My schematics are amateurish as best.

I should have mentioned the breadboard was powered by a 12v, 500ma wall wort. Do I need a regulating circuit to feed this driver?

You need to tell us exactly which stepper motor - its current and resistance ratings and number of wires at least.

Sorry, the motor is rated at 10~12v, 259ma, 7.5 ohm., four wires.

I pulled the SN754410 and replaced it with a driver circuit with the same result; even with a heat sink, it is going to fry. The motor is also running hot. I need the 8amp, 12v supply to run all the devices on this project (3 servos, three DC motors, two steppers, LED's, etc.). The steppers work with smaller wall worts, 12v - 500ma, but not the larger supply. Here is a rough drawing I am hoping there is a simple method of limiting the current from the power supply for these steppers? If not, is a separate power supply possible, using the same Mega?

ifugaopapercraft: Sorry, the motor is rated at 10~12v, 259ma, 7.5 ohm., four wires.

7.5 ohm at 260mA is about 2V, not 10 to 12, and given its 4-wire its likely to be designed for chopper-drive, so ignore the voltage rating and use the 0.26A as the per-phase current. A chopper drive like the A4988 is the recommended way to go, not an H-bridge, in that case.

But first measure the winding resistance to make sure it actually is 7.5ohm...

Sounds like quite a small motor at 0.5W static dissipation...

One way to avoid the need for a chopper-drive circuit is to use series resistance with the H-bridge. Since The SN754410 will lose at least 2V, we'll call the supply 10V which means a total of about 40 ohms is needed per winding, so 33 ohm loads in series with each winding will make it compatible with 12V H-bridge (but will dissipate 2W each - so use 3W rated power resistors). This technique is the "poor-person's" constant-current drive...