Volvo 740 Turbo 90 Cuts off on start

211,095 Mi, 4 Cylinder (I think).

Sometimes when I start the car it will not start. Or it will start then seconds later power off. After a while I have to keep trying or press the gas at the same time to get to stay on. Does anyone know the problem? What I need to do to get it fixed?

The battery level is now about 12-13.

I took it to the shop last week, but they couldnt see any problems with the car.

(I recently installed my own store bought radio and front door stereos.)