Arduino UNO restarts when powered via an External power supply

I have a strange problem. I've got an external (AC-DC) power supply rated as 12V and 1.5A. This power source is supplying power to a motor shield "Sparkfun easy-driver" and a Coin Acceptor device. When I connect the Arduino to the USB, everything works perfectly. But, when I try to connect it to the same external power supply by connecting the V+ and V- to the Vin and GND pins of the Arduino, it keeps on restarting by itself. There's also a speaker, SD card shield and a LCD Serial screen connected to the Arduino itself.

I measured the output voltage and it's a fixed value around +-12V.

Any ideas what is the problem?

me saleel siguiente error al compilar por que
collect2.exe: fatal error: cannot find 'ld