nissan micra k12 1.2 se


got a problem with my car

its a 54 2004 plate micra 1.2 se 3 door manual

the problem that i have with it is everytime a do a cold start in the morning, the car will start fine and idle a bit high like normal
then the revs will start to drop and then go back up again, the car has never cut out

it will only do this untill the car has warmed up, once worm the car doesnt want to cut out

the car has 90000 miles on it

i replaced the petrol camshaft oil sensor about 40000 miles as this had gone, the car would just cut out unless the throttle wasnt applied
i know everyone says timing chain but obivioulsy when i swapped the sensor the problem went away
i have had no dash lights on

could it be the RPM transmitter sensor that has gone or the air flow sensor
or is it the timing chain, if it was the timing chain i would have thought it would do it regardless if the car was warm or not

also the fuel enconomy is poor also, 100 miles for 20quid if im lucking

should i change the 02 sensor,lamba.or the fuel injectors?

i hope i make sense, many thanks for your help

in advance