Why does my car slow down by itself when driving after 10 minutes while I am still pushing the gas pedal?

Timm, take the car to an auto parts store and ask them to check it for computer trouble codes even though you said the CEL is not on. Not every problem will turn the light on and there may still be some codes stored in the memory as history codes. It sounds like an engine system management problem so there code be a helpful code stored there. Let us know if there are any and what they are, in the exact order they appear. It is also possible that the bulb is burned out and that is why it is not lit up. Have you watched the CEL light when you turn the key to the on position before starting the car? It should light up, then go out after a few to 30 seconds or so after the car starts.