Q: When the car hits 40 mph it feels like it wants to stall and slow down.

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Check engine light not working. Cranks slow. When driving up hill looses speed, reaches 40 mph and wants to slow down. Once it goes under 35 mph it picks back up. It looses speed even on flat ground at 40. I have changed fuel pressure regulator and a throttle sensor. Seemed to get worse after changing throttle sensor.

My car has 200000 miles.
My car has an automatic transmission.

This is a sign of a potentially failing or dirty mass air flow sensor. This is a unit that regulates the air intake into your engine and ensures that proper air/fuel mix is injected into the engine. When this is dirty or not working properly, this will cause an intermittent delay or lack of acceleration as you describe. To avoid any unnecessary repairs, I would recommend having an expert from YourMechanic come to your home to take a look at the vehicle and properly diagnose your loss of power.

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