Can someone please help me!

I have a 2015 Mitsubishi Mirage ( 3 cylinder car) it has 93,000 miles on it I bought it with zero. Lately I noticed usually on longer trips on the highway if I reach speeds of about 65 -70 mph (3000rpm) my car slowly starts to deaccelerate out of nowhere and no matter how hard I press on the gas my rpm and speed want to deaccelerate until it reaches zero . The wierd thing is when this is happening I can throttle and My cars rpm will respond and my car will accelerate for literally a split second then deaccelerate again so I actually I am able to if I wanted and have before to keep the car going by stepping on gas letting go and stepping on gas very quickly and keep it going over and over to keep it at a constant speed or even accelerate faster. But to fully fix it I must pull over and go to zero and then it will work normal again for however long then when I go to 55 mph it will most likely start happening soon again and I have to repeat the process. When driving around town even doing speeds of 45/50mph the problem will never happen. Please help I already tried fuel injector cleaner 3 times and I used mass air flow sensor cleaner as well… I also have a habit of driving car on empty tank of gas a lot so I know hearing that initial thought might be fuel pump but would that really allow my car to drive perfectly normal around town doing 45/50mph only not on highway? And would I really still be able to accelerate when the problem happenes by rapidly letting go of gas and pressing on it over and over ? It’s very wierd please help!! Thank you In advance!!!

What comes to mind is a malfonction of the throttle position sensor(TPS)
Basic principle of throttle position sensor is to detect the throttle valve opening angle which controlled by accelerator pedal. Then the tps will send this data to the ECU. If the tps malfunction, the engine speed cannot control by accelerator pedal, engine speed will increase or decrease without any press or depress the pedal.
A few other possibilities:

  1. Air flow meter clogged or malfunction
  2. Oxygen Sensor Malfunction
  3. Dirty or Clogged Fuel Filter

or, drive by wire setup? does it have an electric controlled throttlebody?

I’m not really sure. How would I know that ?

Look at how the gas pedal is connected. If it has a lever pulling a cable, it isn’t drive by wire. If it has an electronic output, it is.

Honestly, I’d be amazed if any modern car isn’t a drive-by-wire accelerator. It’s less costly and far better control-wise to put a transducer on the pedal linkage, feed the signal to the ECU, and use an ECU output to control the throttle position that to try to do it the old fashioned way.

Is the check engine light on? Since both the vehicle speed and engine rpms are decreasing at the same time, and for no apparent reason, most likely either something wrong with the throttle control system, or the engine computer is detecting a problem and putting it in limp mode. If the latter, then the check engine light would usually turn on. Besides that, other possibilities are

  • faulty fuel pump
  • evap system fault is causing a vacuum to form in the fuel system
    (both of the above would show up in a fuel pressure test while driving)
  • ignition system fault, probably heat related; e.g. faulty crank or cam position sensor ,; faulty crank position sensors often don’t throw a diagnostic code.

No Check engine light is on maybe I should try a pressure test like you said

Sounds consistent with gas starvation. Ideally, it should be tested for both pressure and flow volume. A restricted supply will have decent pressure at idle but not be able to provide volume as load increases. If you never replaced gas filter and car has one, that’s something to just try as it is cheap.

Yea, I never had a tune up and I do deliveries and put 94000 miles on my car in 2 1/2 years I’m going to get new spark plugs and fuel filter and just do the air filter myself my boss is telling me fuel filter and this mechanic is trying to make me change my maf and fuel pump even tho he doesn’t sound sure I think he just wants money because he also wants to do my tune up and it prob is just the fuel filter