Need help: engine stalls slowly when idling

[Simon930Berlin]

Hello,

so my 77 3.0 starts either at the first turn of the key, not at all for a while, and then again only after I hear the fuel pumps vibrate for a sec. They start vibrating every 10th time or so, very shortly in key turn before the actual starter kicks in. I can hold it there, the longer I hold it the better it starts.

so
1) how come the fuel pump doesnt always buzz at the same position of key turn?
2) why does it continuously loose idling rev very slowly, until it stalls? It sometimes tries to rev up shortly before shutting down, to no avail.

Help!

When keeping the Idle manually at 1000 rpm while driving, the car runs perfect. But that's a little sketchy in city traffic..