lack of power

My 98 F150 has no power to take off, My mechanic put it on the scanner and everything reads fine. Has good compression on all cylinders 150 PSI, fuel pump lists 39 volts. Replaced the master air flow senser.
Idles fines but when you accelerate it sounds like it's not running on all cylinders, takes about 4000 RPM to get it going and then will slowly take off. We are at a loss for what is wrong with it, anyone have any suggestions?

Have the fuel pressure checked,could be a weak fuel pump.It could also be a plugged Catalytic converter,remove a front O2 sensor and install a back pressure gauge.Most auto parts stores sell back pressure gauges.It is posibally one of these two.

I believe my mechanic tested the back pressure, got 2lbs on one side and 1lb on the other. Factory says it should be around 6 lbs would this difference be noticable? What about if someone put something in my gas? I know water it just dies but what about something else? what does sugar act like?