I have a 2001 4.6 liter. It feels like I have an extreme loss of horsepower. It's not consistent, but will do it from take off OR at higher speeds. The timing is right, the idle is fine. It almost feels like I am trying to drive with the e-brake on. The only thing I haven't changed yet is the exhaust system. Truck has about 106,000 miles. Could this be caused by a bad cat or muffler? I already changed the fuel filter. Please help.

 

Yes, it could be a clogged cat. Muffler shops can use an infrared tester to see if this is the case without dropping the system.

 

I assume you have changed the air filter, cleaned your mass air flow sensor, and your gas filter. It could be the fuel pump. Check the fuel pressure.