Sometimes the engine will stumble (maintaining or increasing throttle position has no effect and RPM's start dropping as if it's starving of fuel). It usually happens about five to fifteen minutes after starting and most often when traveling at highway speeds. Removing your foot from the accelerator pedal, allowing the engine to return to idle and waiting a moment allows the engine to return to normal operation. I don't think it is electrical because the engine idles OK and doesn't idle roughly. After a moment the engine has normal operation. The MIL (malfunction indicator light) does not come on. Recently replaced Fuel Pump, Forward O2 sensor and MAF sensor with no improvement. Added a can of fuel system cleaner with no effect. Any thoughts?