Cisco 5.0

I have a 94 Mustang with 215K miles. I think my Tranny is going out on me not sure but i think it is. It runs fine sometimes in the morning but later in the after noon like around 5 when i get off of work it runs like its going to die on me. when i press on the gas peddle it feels like the car is misfiring and going to die out. but after a while the car runs like this for a while not going past 30 to 35 miles per hour. I have notice once the check engine light goes on it starts running fine. And just today i was driving it and i couldnt go more than 15mph couldnt go past 1500 rpm and i had to pull over more than once to rev the engine to see if the rpm can get past 1500 it did but for a while and it would do the same thing. Can anyone help me out plz. Thank you