Henry, I have an almost identical problem with my 89 420SEL
It stumbles/hesitates for a second only after start-up, only with a warm engine, and only upon initial acceleration after start-up. My idle was high with a warm engine like yours but I replaced the idle air control valve which corrected that problem.

Were you ever able to diagnose your problem?
Anyone else have any ideas?

I’m going to run some Chevron techron fuel system cleaner through and change the fuel filter but I don’t know where to go from there.

Thanks,

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73 220D
08 997 C2S
03 Jetta wagon TDi