Lost all power after around 5 minutes once engine has warmed up

[Bongo100]

My bongo 2.5 v6 petrol 1999 automatic has just started losing all power after around 5 minutes of running fine the engine seems to be good but then it starts cutting power intermittently on and off about every two second I thought it was something to do with the ignition leads or distributor cap so I've replaced both of them and it made no difference I've also checked all filters and tubes and all looks good any ideas??

[haydn callow]

Try it with the petrol cap off or loose