There are a host of problems that can cause what you describe. Some things to check:
Fuel filter.
Fuel pressure (weak pump maybe?)
Air filter.
Exhaust restriction.
Low engine compression.
Any of these things can cause a loss of power. Keep in mind that with 300,000 miles, it is possible that the engine has simply gotten too worn and tired to keep up with the big dogs.
Thursday, April 2nd, 2009 AT 5:34 PM