Car wont shift gears or go over 20mph.

My car is a Poniac G6 it has about 192,000 miles on it. A lot
I know. I only use it to driver to school at work (both only 15
min away). I have issues with it speeding up over 20mph.  
In the mornings (going uphill) it usually shifts just fine, but
at night when I come home (going downhill) it wont speed
up over 20mph no matter how hard I press the gas. The
RPM needle thing goes all the way to the 5 and then when I
let off it goes back to 2. And If I press the gas while going
over 20mph it will continue to move from the 5 and 2 when
ever I barely touch the pedal. If I'm going under 20mph it
will speed up and the RPM will slowly increase to the 3,
until it reaches 20mph then it will move quickly up to the 5
and once it reaches the 5 I let go of the pedal because the
number 5 on the meter is all blocked in red and if its red
can't be any good. Btw if you couldnt tell, I know very little
about cars.