I recenlty bought a Honda civic si 1991 (a month ago) it was running quite well but now when i drivi it for about 20 minutes or so and when the motor temperature reaches it's normal point, it begins to cought and after a little while it stalls and I can't start it again after the motor is cold anough. Then it runs as well as a new one but then when it is hot enough it stalls again.

What is my problem and what do i have to do to solve it?

Thanks for your answer