I have a 1990 buick reatta that will start up and run fine for about 15 minutes then it will shut off,let it sit for about 30 minutes and it will do it again.It shows no codes,I've replaced fuel pump,crankshaft position sensor,MAF,ignition control module and the ignition coil pack and I still have the same problem, right before it shuts off the engine will rev up and the check engine light will come on and blink eratically. What else could be my problem?