Issues with low vacuum

[platez808]

My car starts up, idles, and runs fine when cold; but as it warms up, the vacuum at idle drops to about 12 in/Hg from the typical 18-20in/Hg and it gradually runs crappier and crappier, which includes hunting idle and stumbling. This just happened randomly one day about two months ago and am finally giving up trying to figure it out on my own.

I have a series 2 RB25DET in an s13 hatch. It has wiring specialties premade harness, isis intake mani with q45 TB, FMIC, apexi mushroom filter, stock turbo/mani, splitfire coilpacks, ngk BCPR7es spark plugs, 3in exhaust, oil catch can, greddy type RS recirculated, and redid all the gaskets/oil pump/water pump/timing components about a year ago . I have both the stock and a z32 nistune ecu with stock maps as well as the stock maf and a z32 maf, which I've tried multiple combinations of and have had no luck.

I've QUADRUPLE checked for vac leaks by spraying around with starter fluid, replacing all hoses, and redoing my intercooler piping and I feel like there is no way I have a vac leak. I've checked my timing both mechanical and base timing of the CAS. I've checked the TPS voltage. Everything seems to check out and I still have the same issues.

The only thing I can think of and haven't tried yet is a compression test but I'm trying to make time to rent one and do it. Is there anything else I should try??

[platez808]

I forgot to put that I actually deleted the IACV and use the thermostatic idle control built-in to the q45 throttle body, which, once it was dialed in, worked way better than my IACV ever did.

[platez808]

Anyone have any insight?