Mercury Marquis Loss of Power

2005 Mercury Grand Marquis with 20,000 miles. After travelling between 300-400m on the highway, the accelerator begins to get very tough to press. It continues to get worse and gradually feel the speed dropping. With it pressed all the way to the floor top speed may be 65mph. Then when going up an incline the speed will then reduce to 40mph and sometimes may not even increase to 60. Now, I can pull over on the side of the road and shut it off for a minute, or even less, and restart. It will have better power, like top speed of 80mph, which with this V8 should easily be going over 100. And, after 10 minutes of driving will again have the problem of loss of speed/power. Can stop again for a minute and top speed will be back to 80. Up to the 300m mark it is fine, if the accelerator is pressed all the way it would be going 120. Once it sits over night, it is good for another 300m or so. Check mileage and was getting about 23mpg, normal. Have tried setting cruise control, but no luck. Went to dealer about 10 different times and they have ruled out catalytic converter, spark plugs, fuel injection, etc. They put it on the machine for diagnotics with results coming back normal. They changed the fuel filter and fuel pump. When the pump was removed, it had in it very dark discoloration of fuel like that of dark tea. But, the fuel coming from the lines looked good. Made another trip with new fuel pump/filter and have the same problem. The dealer service manager has talked to all(most) other Ford/Mercury service manager and noone has a clue. Thanks in advance for your help.

Inlet air hose collapsing?

2005 was the first year for drive by wire. There is no physical connection between the accel pedal and the throttle plate. Finding the accel pedal tough to press seems very strange, but I don’t know the system. If no one here can help, try

It seems like the dealer should have thought of it, but maybe there is not enough air getting into the tank creating a partial vacuum which will decrease fuel pressure. This might only happen when fuel demand is high, not at idle so they might not pick it up with a fuel pressure test at idle. Note that your mighty V8 is only 281 CID (4.6L) and about 250 HP. The Grand Marquis is electronically speed-limited above 100. but well below 120 mph to prevent drive shaft vibration.

The MAF sensor may be going bad and running the car too rich, choking it out.