Could be many reasons. Start with a basic tune up. The year, make, model and engine info would be helpful.
This is not normal. If your car is doing this, get it checked out.
Might be a vacuum leak
Have your motor mounts checked for cracks or breakage.
There are several things that can cause your Ford E2 50 Van to hesitate while driving. The most common cause is a malfunctioning ignition switch.
When you press your foot on the gas pedal, it tells the car it needs more gas. The engine then has to take in extra gas. Once it has the extra gas, it must compress the gas, so the gas will have a higher pressure. Once the gas is pressurized, a spark plug goes off, causing the combustion, which powers the car. The time it takes to do all of that causes the hesitation on initial acceleration.
The Van Karner is a tear gas gun.
Key on, engine off. Press gas pedal to the floor three times.
It might be a partially blocked fuel injector or low fuel pressure or possibly even a partially blocked fuel filter.
You might have a blown headgasket on top
Sounds like you either need to replace the fuel filter, or your fuel pump could be going out, and a throttle position sensor is possible, yet not likely. I would start out by replacing the filter.
Try pressing on the gas pedal with more force.
Occasionally the gas pedal will have some resistance when you press it. You could check and lube gas pedal, gas cable, and throttle itself.