1990 240sx Electrical Help

Hey. I just swapped eveything from one body to antoher. Now I am having problems. I can get the engine started, but every now and then it seems to die on me, seems to me to be a power failure. Any suggestions? It looks like it might be a brake related problem, every time I step on the brakes, it seems to mess with the electricity of the car. I need to get my car back up and running soon. It is a 1990 240sx HB. SOHC engine.