Focus RPM > 4000 problem

I have a 2000 Focus with automatic transmission and Zetec engine. When the engine is hot everything seems fine till it reaches 4000 RPM at this point the engine will loose power, and for want of a better word it will 'hunt' and will not exceed 4000 RPM. When the engine is cold it runs fine and will go up to 5500 RPM +. On advise from a newsgroup participant I have had the car into the garage and hooked up to the computer, no fault codes. The mechanic changed the fuel filter, still the same, he then did a fuel pressure test pressure it was OK, next he did a flow test and said it was low, and could be the problem, I have a little problem with the flow problem as the car runs good when cold and at $600. to install a new pump I would like to have a little more assurance that the pump is the problem, or who knows where this may end. Any help would be appreciated. Thanks.