94 geo prism stumbles after running about 10 minutes?

alright my geo prism has the 1.6l in it. a while back it stalled on me, i replaced the fuel pressure regulator and it fired up and ran nice until it runs for about 10 minutes then it stumbles on acceleration. i replaced the regulator again due to the fact that it didn't do this before. it made no difference in the way she runs. i did get both regulators from junkyards and yes i know there is a possibility of bad ones that is why i replaced it again. i do think it is odd for two different regulators to make it run exactly the same especially since one of the cars ran when it had a fire so i would assume that regulator is good. i really don't want to put in the 100 dollar regulator for a new one unless i know for a fact that the 4 dollar ones are bad. does anyone else have any other logical ideas of what is going on. the car also has fairly new cap/rotor and spark plugs. i will award best answer to the best logical answer thank you