Fiesta 2003 Zetec crank no start question.

Well, I have some good news.  The thing started today.  I was going back through some things I'd already superficially looked at, and had decided that I should now look at them in more depth.  So I started with fuel.  I took the injectors out and sprayed carb cleaner through them in both directions while pulsing them open.  Each time I tried this, the spray pattern was never much good once they'd dried out.  I pulsed them in an ultrasonic bath for hours, and there were deposits coming out seemingly without end.  I decided I couldn't trust them, and got a new set of injectors (Ridex) off eBay for £40 (which seems to be lower than the wholesale price, for some reason).  I fitted them to the fuel rail and pulsed them into a test tube to be sure that there was no air in there.  It started but died immediately (which was a step in the right direction).  So today I thought I'd check the fuel pressure while cranking.  I had a fuel pressure gauge with bundy connectors plumbed in (no schrader valve, sadly), and a camera pointing at it to record any drop in fuel pressure.  And this time is actually started.  Twice.  Now, all I have is a problem I had before it all got worse.  At idle (and after blipping the throttle), the RPM goes so low every few seconds that it almost dies (in fact it sometimes does).  In traffic, this is very alarming.