Car is not delivering power as it should and in a seemingly random fashion, unless I floor it, then it seems fine. What might be causing it?

When you change plugs or fuel/air filters, the PCM/computer has to relearn how it adjusts fuel trims for optimum performance with the new parts since those items may change how fuel pressure or air is delivered or spark plugs fire. It may take several startup/warmup/driving cycles or more for the computer to recalibrate it's settings for differing operating parameters so just bear with it for a while. But also, check for any vacuum leaks from cracked or loose hoses which may have been disturbed during the tuneup. If the power lag continues after several driving cycles, then you may have other issues which need to be checked and diagnosed. Possibly a lazy TPS sensor or dirty MAF, etc.. or some other sensor is not giving proper readings simply due to age.

In fact, I'd also clean the MAF and the Throttle Body given the age of the car...they can get sooted up with carbon/dirt which can affect proper airflow readings and reduce performance over time. Pretty simple to do and parts stores sell special spray cleaners for cleaning them.

Edit : Faulty or erratic voltage readings from the TPS can indeed cause power lag/stumble issues and may occur randomly and not trip a trouble code. They can be tested and various sites show how to do it . What you're mainly looking for is smooth increases in voltage readings as the throttle is slowly opened from closed to full open. If the readings are erratic in some spots, then the TPS is bad and is sending fluctuating voltage readings to the PCM for a given throttle plate opening which can cause erratic power/lag since the PCM is trying to adjust fuel trims for erratic or incorrect throttle opening as read by the TPS. Erratic voltages may also not trip a trouble code. One common trick/temporary fix is called "wiping the pot" used in various electronics repairs to remove carbon buildup from potentiometer contacts such as used in a TPS. With ignition off, rotate the TPS /throttle from fully closed to wide open several times in rapid order. Retest and see if voltage readings smooth out. If possible, spray some contact cleaner inside the TPS housing to clean any carbon buildup on the inner contacts. It's fairly easy and cheap to replace the TPS so that may be best if it's bad.

Testing TPS info....may differ from your model...check other websites if needed.

http://easyautodiagnostics.com/nissan/3.0L/throttl...

https://www.google.com/search?q=2000+nissan+maxima...

If you got a P0340, your camshaft position sensor is likely shot. None of that other stuff might actually have needed replacing.

But no check engine light, right? That makes my top suspect a carboned EGR valve, not closing properly until you exercise it with full throttle operation. If the EGR flow is too low it will light the CEL, but usually not if it is too high. Not familiar with the vehicle but in most the EGR is not hard to remove and visually inspect. They can be cleaned or replaced.

If it's electrical, disconnecting the battery sometimes works. If it's mechanical, I would guess the fuel pump. Another weird thing it could be that actually happened to me is bad fuel.