1990 nissan 300zx NA loss of power.

This is my son's car. I noticed last fall that it was missing pretty badly before we put it up for the winter. This spring, I would not start at all until I "woke it up" with a little sniff of ether. Now I'm back to where we were last fall. If I start it every few days, it isn't too hard to start. But after sitting for a couple of weeks it is impossible to start without ether. Once started, it runs rough up to about 4,000 rpm before it smooths out. I've replaced the plugs to no avail.