Ford Repair: 97 Aspire stumbles on acceleration, throttle position sensor, temperature gauge

Question
Hi Shaun,
I've got a 1997 Aspire 1.3L that was running beautifully. Out of the blue, it now stumbles badly on acceleration, but only after it has been driven 15 or 20 minutes.  The check engine light has never come on, so I don't have any code to check.  The temperature gauge has never gone real high, like it probably would have if the catalytic converter was clogged.  I replaced the obvious...plug wires, distributor cap, and rotor.  Installed new plugs, too, even though the old ones had less than 3K mi. on them.  With the exception of one time, it has started willingly and idled fairly well.
I'm not thinking along the lines of fuel pump, or it be stumbling even when the engine is cold.  I'm stumped by the fact that this occurs so long after the temperature gauge tells me it is up to normal operating temp.  What can I check next?  Thanks for your help.  

Answer
Hello Pam,   
    It sounds to me like you have replaced the parts that would most likely cause an ignition misfire, so it is possible that the cylinders aren't being fed enough fuel(too lean). The problem seems temperature related, so it could very well be the fuel pump. It also could be a faulty throttle position sensor, where many times it wont set a code. If you can verify proper fuel pressure, I would say thats the best place to start now, unless you can find that the problem is still in the ignition system(faulty ign. coil possible also). I hope this info. can help you.  Good Luck.   Shaun.