1.4 Petrol - Poor idle, now misfire

[jharrawa]

I hope someone may be able to give me some pointers on places to start with my 2004 1.4 petrol Berlingo.

For a while now it's had a bit of a rough idle, then it started cutting out randomly when stuck in slow moving traffic (It was fine driving around town, you had to get caught in traffic for 5 minutes).  It would always start up again on the second turn of the key, and be fine.  It only seemed to occur when the fuel level was at a quarter or less.  Running around with a full tank has helped, and I've tried some Redex which seems (and I know Redex has it's doubters!) to have improved the idle.

Yesterday and today, when cold, it idles fine but lacks power at low revs when you pull away.  Seems to be pretty much OK when warm.

 So I'm not sure if I have one or more problems, and really where to start looking.  New plugs going in at the weekend, but are there any other things that I can check?  I don't mind spending some money to replace the most likely cause, but I didn't just want to throw a new lambda sensor (as an example) on if that isn't even likely to be the issue.

 Thanks,  Jon.