Had a service in December, fine until now

[Connorsolle]

Hi in December I had a big service. Had everything changed
It was all fine until about 1 month ago, I’ve videoed the problem and my mechanic has plugged it in and no problem has shown up. The problem is when I start the engine the revs drop to 600 and the. If left for 6 seconds it goes up to the correct revs. If I was to turn the engine off when the revs are at 600 and then turn the key again the engine will start up perfect! Preferably the problem is cold starts but saying that I stopped for 30 mins the other day then it started again
It’s not every day it’s 1 in every 7 times I drive so it’s not common, I’ve just had the battery changed, no difference and also had the fuel filter changed aswell! Still no different I’ve even just put £50 in my tank of unleaded, yet to be any difference, seems to only happen when my petrol level gets below 100 miles left on the tank but I’m not for certain on that. I need to get this sorted as I’m worried the longer I leave it the more problems!
Thanks
My names connoe with a black 2009 tsi scirocco! Lovely car!!

[Connorsolle]

Ps it drives like a dream so no performance problems just start up! Thanks