Car's accelerator when pushed down all the way doesn't increase and car decreases speed

so whenever i go on long trips mostly over 2 and a half hours the car will be
fine, but whenever i slow down or accelerate really quick it makes a weird
whirring sound. additionally, after going at speeds of 75 or so for multiple
hours i will push the accelerator down all the way and the rpm's won't go up
only down because the car is getting nothing from the accelerator. i decrease
in speed immediately, its almost like the accelerator is worthless at that point.
from that point on, its scary because you feel like you have no control because
the accelerator won't work.. I'm no car guru. any thoughts??