Focus servo issue

Hi all, I have a 2011 MK3 Ford Focus petrol 1.6 Ti-VCT. When I start the car after not being used for a few hours, it starts misfiring, chugging and spluttering (and has a strong smell of petrol) for about 1 minute while it’s sitting on the drive and then all of a sudden the revs will go up to about 1500 RPM and then drop back down to a steady idle. Now while it’s chugging there’s the odd pop from the exhaust and if you put your foot on the accelerator you don’t get nothing and if you do it just drops the revs and stalls. The brake peddle is rock solid while it’s doing the chugging and until it sorts itself out it will stay solid then just go normal. 
 

Every-time you press the brake and come to a stop it starts chugging slightly and if you pump the brake while idling it will start chugging a little bit.

At first I thought it was a bad servo however after doing a vacuum test on the servo it holds vacuum and I’ve put a smoke machine on the car and it ain’t coming out from anywhere apart from where it should do. 
 

Has anyone got any idea what it could be or what other test I could try to try and diagnose the problem and get it sorted as it’s getting embarrassing and annoying when I have to sit in my car for ages while it’s shaking the car and popping before I can move off.

Thank you