i dont lose pressure when i turn the wheel. heres how it happens..

1.pulling up to the redlight, slow down, downshift and brake into 1st gear
2.stop completely
3.the idle moves around when sitting (not messing with A/C or steering wheel)
4.the idle drops to about 400-500 RPM's, so I give it a bit of gas (around 1k RPM's)
5. once i let off the gas, the idle will drop to almost nothing (100-200-300 RPM range)
6. it goes back up and sits around 600 for 15-20 seconds or so, then back down
7. repeat

the oil pressure light comes on after step 5 obviously, and kouki crisp was nice enough to explain it to me. im 90% sure its the IACV at this point which i'll replace. I was also told to replace the intake manifold, any reason why and/or if its neccessary? thanks