not sure if this might be timing related or fuel delivery issue or maybe something else...

when i rev the engine in my 540 and keep it steady around 2500 rpm, i get a fluctuating (varying intensity) vibration - somewhat like if I were to turn on the AC - the engine seems to be losing varying amounts of power. this vibration occurs even when I have the clutch pedal pushed down (transmission disengaged) so I can eliminate the transmission as a cause.

i would describe the varying vibration intensity as a regular sine curve - the intensity slowly increases then decreases then increases over and over. and the 'vibration' is much like

I'm not getting any error codes right now. I recently replaced the PCV cover, 02 sensors, intake manifold gaskets, oil change, oil filter and fuel filter. and there don't appear to be any vacuum leaks as far as I know.

can anyone make any suggestions? could irregularities in fuel pressure cause this? what about coolant or air intake temperature? or even power steering pump perhaps?