Briggs and stratton lawn mower engine problem

Hi,

I have a lawn mower with B&S engine, one with the metal gas tank/plastic carb with primer, one like this:
YouTube - How To Replace The Diaphragm On Briggs And Stratton Primer Plastic Carburators On Metal Gas Tanks
The problem with the engine is that it shuts down all the time. When i first start it i can ride for a few minutes and then it shuts down. The thing that happens is that the primer gets pushed in when the engine is running (something creating a vacuum?). Then i have to wait a minute or so until the primer gets out again and then i can start it again. This keeps repeating itself, but the time i can mow the lawn gets shorter everytime.
I have already cleaned the carb etc and the diaphragm is still ok.
Somebody knows what this problem is?

Regards,

Senne