I have a D1 that I use occasionally and the rotary switch that controls PC, Play, S, C, and Self Tmer works correctly in only the Play mode. When I put it in S mode it reverts to Play mode, when I put it in C mode it reverts to the S single shot mode and when I put it in Self Timer it reverts to the C continues mode.
sounds like your custom functions need to be looked at. You can tell C mode to fire 1 FPS, but not sure about selftimer.. yo umight want to dig up the manual, or look on dpreview.com for the custom function guides..
Your D1 is f***d . Get a better body . Servicing an old body like that, to be honest, is not worth it, unless you're married to it then you need some divorce papers ...sorry.
I own a D1 as well and now it is one of my exotic paper weights , it has so many weaknesses that it made me find so many ways to make the D1 take beautiful images when it really can't , which in return made me a tad bit smarter...lol.
. Get a better body 
. Servicing an old body like that, to be honest, is not worth it, unless you're married to it 
...sorry.