The difference between Pro and Amateur cameras often comes down to shutter mean actuations to failure design and resistance to handling, temp extremes or other factors.
The FM3A is not a Pro designed camera as also evidenced by its use of the MD-12 Motor which had its start with the original Nikon FM, I believe.
Now its a fine Motor but certainly not designed for very heavy use. I have 3 and two have experienced various problems over the years. I also have an FM3A, a fine camera but I purchased it for the hybrid shutter, which I thought was clever engineering and a logical progression to my Nikon FM2.-Dick