there are a few known causes of idling style issues. dirty contacts (AFM or TPS), worn out valves (AAC valve), and other wear style issues (wiring, coil packs etc).
these would typically result in unstable idle, fluctuating idle, or blipping style idling - the r33 service manual would list several fixes and corrective measures for this, you should download it and have a read
the engine rpm dropping to 500rpm (almost stalling) on clutch in is usually associated with an atmo bov
its been covered to death over a 100 times, its always usually the atmo bov, it dumps air that the AFM has already measured
so the mixture is out of scale, as the air is missing so the engine overstalls and rpm's plummet