1997 Hyundai Elantra fuel injection not working

I'm a poor college student and I have no other car options and no one can answer my questions.

The car is having fuel injection problems.
replaced fuel filter, plugs, wires, coil, there has been more done to the lines, but I'm not sure what.

the current problem:
the check engine light has been on for 4 months now

the car will run and then when I park it to do errands or whatever, it will not start unless I let it sit for 30+mins.

just 4 days ago this problem worsened when I stopped 1.5hrs into a 2hr drive at a rest stop and then restarted the car to find that it would not go faster than 30mph/3rd gear. After sitting on the highway for 40+mins the car started running like it was before it died.

I really need someone to help me with the problem. Thank you!