Problem with Ford Mondeo acceleration?

my N reg mondeo suffers from terrible acceleration, i spoke to my mechanic and he agrees its dangerously bad but doesnt have a clue how to solve it, every few days i open the bonnet and rev the engine from under the bonnet if i do this and rev it until it reaches 6,000rpm the problem is instantly fixed and the car has excellent acceleration but this seems to wear off and the next day the car will be under performing, anyone had this problem before or know what could possibly be wrong, a friend suggested it could be air diaphram