Focus stalling when stopping

[bulley]

Wondering if anyone can offer some help.

My 2012 Focus Mk3 stalls all the time, usually when coming to a stop, also after reversing. I can usually make it do it in idle by giving it gentle blasts on the accelerator. 

When I give it a gentle blast on the accelerator I can hear an air burst from somewhere. The engine is also running rougher, noisier and acceleration is more sluggish when it has been raining. 

I've checked the engine for signs a water, the spark plugs/coil pack look dry but I can see some signs of wetness toward the back of the bay, in front of the driver's position.

I've taken it to the garage twice and they couldn't find the problem although they acknowledged the sluggish acceleration, they fitted a new mass air flow sensor but that hasn't helped.  I want to do some more diagnosis before trying again with someone else.

Thanks