85-z28

Hi, I have a 85 z28 with the stock ERS delco radio/cassette player in it. It has never been changed or messed with. My problem is that it won't turn on when I push the power button. However, if i leave it in the "on" position in about 20-30 mins it will just start working and get horrible reception. I am not very experienced with radios so i have no clue where to start looking or even how to.