I'm gonna asume your S2 is MFI, in which case, they run lumpy at cold start. Always do.

Basicly is the fuel not vapourising enough in the cold air to burn effectivly.

The revs will be higher anyway at cold start, usually about 1300rpm. But my old one used to bottom out and twitch like a mo fo.

EFI ones shouldn't at all.

The only reason I ever disconnected my cold start injector was because it used to flood the engine all the time.

So chuck in some injector cleaner and see what happens. If not take it to your local tunup shak and get you injectors cleaned and the flow rates checked. You shouldn't have to replace injectors, they usually just need a clean.