Low Idle Issue 5.3L LM7/4L60e

[66LSx]

Alright guys... a bit of an issue in my 66' C10... but first the layout of the land

2004 5.3L LM7 / 4L60e
DBW, stock internals/stock tune
CAI/Full Long Tube Headers (w/ front o2's)
Fuel Pressure gauge is showing 58 PSI without deviation (Stealth Aeromotive 340)
Feels like it runs out/drives fine... don't have enough miles on it to get a grasp on MPG (300 miles only so far)

Upon coming to a stop, RPM's drop 400 or even below. Occasional sound of backfiring (when I simply let off the gas...stops after brake pedal input to TQ)

VSS is working (says my speedo) but I had to add a pickup resistor wire (10k ohm) like I did with the tach signal from the ECM

No MIL codes being thrown or presently being stored

o2 voltage looks good via tech2 same with MAF and injectors

Seems to improve after a long temp increase... but is still even present in both open and closed loop system. o2's are brand new delphi's... along with a new delphi MAF.

Using a vette filter/regulator combo after the Stealth 340 to the stock returnless system.

Issue came about 100 miles after first drive.

Have a vacuum gauge installed in the truck also... idling @ 17-19"hg @ idle.
Carb cleaner couldn't sniff anything out so far (haven't pulled the manifold cover off to check EVAP solenoid cap or the old booster cap.)

Not sure what the truck LM7 5.3L should be idling at in regards to "hg

It hasn't stalled... but it has given a shimmy and touched that 250-400 RPM range

clueless

Ideas?