i have a seemingly bizarre situation with my engine. it starts great and idles just fine in park or neutral without shaking or rough spots and is fairly quiet due to a recent valve adjustment. when it is warm the car drives just fine and accelerates great and isnt lacking in power at all, however, when i pull up to a red light with my foot on the brake, it starts shaking fairly violently and will occasionally stall if it is also still cold. this problem is easily remedied by popping it into neutral to raise the rpm a bit. what the heck is wrong?