It is almost certainly due to old grease in the mechanism that transfers the shutter release movement from the body to the lens. The shutter itself is not at fault and does not need to be touched. It is an easy fix. The dead giveaway is that it will fire if the lens is rotated/jiggled on its mount.
I've posted here on APUG regarding what it is and how to fix it, but I gave more thorough instructions here:
This regards an SQ series lens, but the ETR series lenses have mechanisms which work the same way. I first discovered the problem and figured out the solution on an ETR series lens, a 75mm E-II in fact, so be assured these directions work for ETR lenses.
You will only need to remove the back plate to gain access to the places you need to clean and relube.