If your vehicle is showing noticeable loss of power when driving up hills or under load, here are the most likely causes and the parts you'll need to fix each one.
Under heavy load, the engine requires a stronger spark to ignite the denser air/fuel mixture. Worn spark plugs or weak ignition coils will fail to fire, causing a noticeable stutter, hesitation, and loss of power.
A severely dirty air filter restricts the volume of air entering the engine. When climbing a hill, the engine needs to breathe heavily; if choked, it cannot produce adequate power.
When driving up a hill, the engine demands a higher volume of fuel. A clogged fuel filter or a weak fuel pump cannot deliver enough fuel to keep up with the demand, causing the engine to starve and lose power.
The MAF sensor measures air entering the engine to calculate fuel needs. If it is dirty, it underestimates airflow under heavy load, resulting in a lean fuel mixture and a sluggish, underpowered engine.
If the engine RPMs increase but the vehicle doesn't accelerate up the hill, the transmission is slipping. In manual cars, this is a worn clutch; in automatics, it can be low fluid or worn internal bands.
In forced-induction engines, a loose hose clamp or cracked intercooler pipe allows pressurized air to escape. The engine loses its boost pressure, resulting in a dramatic loss of power under load.
A restricted exhaust system acts like a potato in the tailpipe. The engine cannot expel exhaust gases fast enough under heavy load, severely choking engine power and causing it to bog down.
Worn piston rings or a blown head gasket prevent the engine cylinders from holding pressure. This results in a severe, constant lack of power that becomes highly apparent when the engine is stressed on an incline.