Hello all, I am new here.

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My 97 altima recently had idle problems. The idle of the car was normal (~750) when I stopped before a red light and AT was at "D". However, if I
applied any load to the engine at this time, for example, turned on A/C, turned the steering wheel, press the PowerWindow button, the idle dropped to 500 and backed to 750 in about 0.5s. At mean time engine vibration got worse. It sounds to me that ECM tried to maintain the idle at 750 and sent the instruction, but there was a hesitation for the idle to stablize at that point. I also noticed that in local run (30~40mph) if I push the brake slowly, the ilde also drops to 500 and backs to 750 in 0.5~1s delay.
The car just passed smog test in CA and the emission output was very good - emission polluents well below average.
I tried to fix the problem by a major tune-up, in which i changed spark plugs, wires, distributor cap, air filter and fuel filter. After the tune-up the car was more powerful, but the idle problem was unresolved.
Any ideas?