Low/no feul pressure

I have a 2000 taurus, 113,000 miles that is no getting enough feul pressure. I have had it to 4 mechanics and still unable to resolve problem. When driving around 40-55 mph it bogs down, acts like it's running out of gas or it will stall while driving down the road and u have to pump the gas pedal to get it going again. Also sometimes it will idle and sometimes it doesn't weather it's in park or gear and sometimes it doesn't want to start it will just turn over or start for a second but die cause it's not getting enough feul, you can start it with starting fluid though. I have replaced the feul pump and filter twice, the relay switch has been replaced, plugs and wires and injectors cleaned. Please help! Thanks for your time in reading my question