Arduino nano crashes

im trying to make rgb fader, but my arduino nano is working against me - it randomly resets itself, but only if i have it connected over onboard usb. Voltage is 4.6V. But if i connect it directly to power supply and srt the voltage to 4.6V it works with no problems. I tried running Blink with onboard led(pin13) and it also crashed. if i connect reset pin to +5V without any resistor when arduino is already running it doesnt crash.

Where to look for problem ?

Anyone ???

I also noticed that it only crashes when serial monitor is closed, even if sketch doesnt use serial. Itried to remove 0.1uF ceramic cap between FT232 pin 2 and arduino reset pin and it works without problems. i just have to do manual reset