87 626, runs not good

[cavyman]

I have an 87' 626, and it only runs good half the time. You start it up and it seems to run good but 20min-2hours, it start to overfuel, missfire, if you try to rev it up quickly, it will die and the not start for 20-30min, it you rev it up slowly it will only rev to about 3500-4000 rpm then starts to sputter out, while it's running like crap i've checked and it has spark in all cylinders, i've replaced the computer on the advise of a local mechanic because while it is running like crap it doesn't put out any codes, he also replaced a couple sensors and the distributor cap. i'm running out of ideas and money, any body who could help it would be great, and when it runs good it runs great!!, awsome fuel economy