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Acceleration problem

I own 2000 Hyundai Elantra with 120K miles, manual transmission. I have a problem with acceleration. From time to time my car has delayed acceleration. As I increase the speed somewhere between 2000 and 3000 rpm the steady acceleration rate drops suddenly, as if I took my foot of the gas for a short time. I push the pedal to the floor but the engine does not react. The car accelerates but slowly. As rpm gradually pass 3000 mark the car suddenly picks up the normal acceleration rate. I don’t have any other issues with the car. It starts perfectly and it idles fine. Acceleration problem does not happen every time and it seems that it is more likely to happen when its warm outside or when AC is on.

I took care of the vehicle. I change oil and filter every 3000 miles. I checked spark plug wires, resistance is normal. I checked TPS, resistance is ok. I changed fuel filter and fuel pump 6 months ago. I changed spark plugs 2 years ago, I installed those that do not require gap to be adjusted. I don’t know what else to do. Can O2 sensor be a problem? MAF? Fuel pressure regulator is also about 1.5 year old.