1990 Trooper 2.6 Failing NOx

Howdy Everyone, I've a problem here maybe someone with a similar thing can help fix: My 1990 2.6 Trooper (air, 5sp) has failed the test-only here in CA a
couple of times. HC passes at 53/48 ppm. CO passes at 0.19/0.20, but NOx is 1189/1096. The Cat conv has less than 1000k, the head gasket was replaced just before the smog test and I cleaned all the EGR tubes and ports and sensors. Timing is right on.
I took it in for repairs and the smog check guy couldn't figure it out. But here's what he said: The EGR valve, though good, doesn't seem to be staying open long enough. The vacuum is quickly dropping off, closing the valve too soon. I've checked all the hoses, etc, and it's all good. Thermal valve seems to be good, though I haven't removed it to test it. His suggestion was to replace the EGR valve and Cat... I think he's just shooting in the dark...
I have a concurrent problem that may or may not be related: Idle is too low on warm up... Cold start enrichment is fine, truck idles fine for a minute, but once it's slightly warm, until it is fully warm, it idles too low, stumbling. Warm idle is perfect. Smog guy said vacuum to the pressure reg is also dropping off, but the VSV is good. I don't see where the EGR and fuel pressure reg vacuums are connected, except at the throtle valve vacuum ports...
Any suggestions or more information?
Pulling my thin hair out, slowly
Alf.
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I have a 1994 Isuzu Pickup 2.3L manual.
As what I read from Haynes Manual, the emission control subsystem you also want to check is Air Injection Reactor System which provides favorable condition to burn the emissions by providing oxygens.
Spike Strip wrote:

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