I have a 2001 Ford E-150 cargo van with a 4.6L engine.It sputters between 25 and 55 miles per hour.I have had at least 2 fuel injection cleanings in the past but,still sputters.I have taken it to the computer diagnostic but,comes back clear.I am out of answers for my van.Someone please help!

Could be low fuel pressure or obstructed air filter.

check rpm and co after the fan kicks in

Sounds like who ever did the work on your car needs to go back and check the plug wires to see if they are going to the right plugs. and check the fuel filter, and fuel pump pressure. I would also check the Catalytic Converter to see if it is stopped up, if so replace it. Check the throttle position sensor with an ohm meter, if bad replace it. Check the oxygen sensors as well. I am running out of Ideas, so I hope this helps.

have you done the proper services , like fuel, air filters, spark plugs at the specified miles? if so is teh check engine light on? if it is have it scan it will give the fault code.

fuel plump or filter

It may be due to plugs, fuel filter, fuel pump, or mass airflow sensor. Is your check engine light on?

57k miles on a '94, nice...
The transmission should be able to handle a 500lb load and a 500lb tow, you really shouldnt hardly even notice the weight unless you going up some steep grades.
as far as what to look at before going on a long trip, diffenetly get a oil change if you havent in 1500'ish miles (or get one on the road when you get closer to the recommended mileage of 3k miles, which you should hit easy enough on a 11 hour drive to your destination) Perhaps plugs and wires if you havent had at least 1 tune up since driving off the lot. all your fluids, bulbs, fuses(bring extra fuses with you), at least 1 spare tire, tire pressure(35-42psi), and anything else that is general maintance that needs to be done also.
Other then that, you should be fine... Just dont forget to check the gas gauge once in awhile, and she shouldnt die on the way :P

have you had the throttle cleaned? Sounds like your fuel pump may be getting hot and cutting out. Its possible it could be a faulty ignition module,but that usually just causes a no start not a suptter and stall.

No fuel pressure, bad pump or filter. check pressure.. clogged injectors. bad ignition module. Because you siad TPS I assume this is fuel injected, ?? Bad wires or distributor or rotor or cap

most likely a fuel filter problem. try takeing a rubber mallet {not steel} and pound on the bottom of the gas tank. this will sometimes dislodge whatever is clogging the filter screen. Your engine fuel injected and needs a certain amount of pressure to operate correctly. You can have a repair shop do a pressure test on the system.