car fails to keep the revs at start

[DanielDefoe]

my car is Renault Megane convertible 1.6. When I try to start it it would start and after 2-3 seconds the engine dies,because it could not keep the revs.I need to rev it up and drive away straight away otherwise the engine would stop.Once it reaches the normal working temperature it works fine and keeps the revs normal in neutral gear.Also it sometimes makes a grinding noise when i initially push the start button,strange plastic rattling and fails to start.That's about 1 in 5 times.
I've been with a mechanic and he connected the car to a computer but it did not show any faults.He still advised me it could be a fault throttle body or lambda sensor he said it could be an electric fault which not necessary shows on the computer).I've changed spark plugs and cleaned the throttle body,but it did not affect the faulty work of the motor when cold.I personally thought it could be a faulty pencil coil,but today i have read about dephaser pulley and got confused .Anyone any idea?
Thanks in advance,would appreciate any advice.

[Stranger]

Sounds like dephaser, change it or get it changed pronto.