need help tracking down engine issues

[cdlong]

for the most part my car runs great. it has some engine issues that i think are interrelated. i'll give you all the symptoms first, let me know if you think they all stem from the same problem and how i can fix it. btw, it's a '95 240sx with a KA, intake, header, test pipe, exhaust.

symptoms:

check engine light, O2 sensor, i was told it was replaced right before i bought it, but you never know.

the idle is rough when the engine is warm, the idle speed dips to about 500 rpm ever second or so, only for an instant and then it jumps back up. i actually noticed it corresponds with a really quiet click under the dash like you turn the brights on and then off.

the exhaust pops when coming off the throttle at high RPM. like pop pop pop, really fast.

any ideas? i want to get this figured out before i put on my new high flow cat in case there are fuel issues or something else that would ruin it.

[NateDogg]

IMO you can't trust anyone, especially someone thats selling a used car. Test the O2 sensor. If you have a multimeter you can do it. Or you can go put the computer into diagnostic mode and do it that way. The popping sound is coming from the test pipe. Thats normal, although you may have alternator problems if the lights in your dash dim and the idle drops when you turn on the headlights.

So check the O2 sensor before you replace it, check the battery voltage when the engine's running without any accessories on and then for God's sake put a cat in that thing.

[cdlong]

i went to autozone to pick up some parts for the weekend and had them run the code again to be sure. either it's different or the first guy told me wrong or something. it's P0400, EGR malfunction. i can see that causing the weird idle. any other ideas?