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Replace TPS (Throttle Positioning Sensor) It's a rheostat that controls throttle. After time and mileage it develops hot spots/wear in areas that get most use. In your case the 45-55 mph area of rheostat. Your description is spot on to symptom of bad TPS.
I would change the spark plugs, spark plug wires, distributor cap and rotor, fuel filter, check and change air filter if dirty, check timing, check and top off all fluids, check or change transmission fluid, check brake pads/.shoes and drums/rotors for wear and check tire pressure, check the condition of all belts and hoses and replace as needed, check condition of radiator antifreeze and top off or replace as needed.
Stuff like: fresh spark-plugs, air-filter,new engine-oil, oil-filter, fuel-filter, plug-wires, rotor/cap, ( some adjustments may be necessary to the throttle-body and the throttle-body and choke cleaner may need to be sprayed inside the throttle-body).
If only under a load, it is possible that the fuel filter is mostly clogged. Also check air cleaner (you probably already did). The concept is that under a load, your throttle body is mostly open, requiring high volume of air and fuel flow. Restrictions in either of these will make it run bad.
Check your power steering fluid level and if it has never been replaced you may want to. The tricky thing about replacing the power steering fluid is that it does not have a drain plug for the fluid. What I have done in the past was to siphon the fluid out with a turkey baster from the dollar stop since it's just going to be discarded, then replace the power steering fluid that has been removed. The serpentine belt may need to be replaced also.
The fuel economy could be cause by many things, but the common is dirty MAF sensor, throttle body, and air filter. Clean the MAF sensor with a can of MAF sensor cleaner and the throttle body with throttle body cleaner and replace the air filter. As for the MAF and throttle body cleaner, just follow the instruction on the cans or ask the parts store for advice such as Auto Zone. Auto Zone can give you the step by step instruction do getting the job done.
Repcap: Check the power steering fluid level and try to replace some of the fluid.
Replace the serpentine belt if needed after inspection,look for cracks on the inside of the belt where the ribs are.
Clean the MAF sensor and throttle body
Inspect and replace the air filter if it's been in there for 1 year or 30,000 miles.
Good luck and hope this helps.
I had a similiar problem on my 1999 Dodge Ram 1500 Conversion Van. It ended up being a partial collapse of the catalytic converter which didn't allow adequate exhaust. The problem only showed itself because I could not get over 55 mph going up a small hill.
I assume you have checked the transmission fluid. Try changing the fuel filter. You wouldn't believe how often people confuse transmission problems with fuel filter problems. The engine is starving for fuel, and therefore, power, and the transmission does everything it can to try and find it.
If there are no lights or ECU codes, esp the water temp sensor, then the usual remedies are to use injector cleaner, and clean the throttle body, check for water in the fuel filter (and the fuel tank). Make sure the engine oil level is full, and so on. I assume a hill outside Sydney doesn't get enough altitude to be an issue, unlike a hill outside of say Denver, CO that goes up near 10,000ft above sea level where you should expect a significant power loss.