Arduino MEGA is resetting when USB connected

Hello,
yesterday my original Mega 2560 R3 started resetting itself, when USB is plugged in. It does not depend on what USB cable I use (I've tried two of them) nor what sketch is uploaded.

Resetting is irregular. I was thinking there is not a much power, so I'm using DC power adapter, but it's the same.

When an USB cable is disconnected, Mega works okay.

L-LED lights very faintly and light changes when I touch USB connector. Maybe wrong contact of USB connector?

Do you have any suggestions?
Thanks.