1995 E320 W124 M104 idle / stumble woes

[Hirnbeiss]

Hi all you Bosch HFM fans. I used to sit smugly as my beast purred smooth and faithful. Now that she's rolled over 159K, though, she has begun acting up.

Symptoms:
1. Oscillating idle (not all the time), sometimes leading to stall.
2. Occasional stumble at speed.
3. Exhaust smell. Sometimes sulfury, sometimes just rich.

What it is not:
1. EGR valve or pipe. Checked w/ vacuum pump, and cleaned pipe.
2. Fuel pressure regulator. Replaced as a cheap attempt after reading an MB recommendation.
3. Oxygen sensor. Replaced. Old one was black & sooty, leading me to suspect rich running condition. After replacing, engine seems more hesitant on acceleration, as if too lean.
4. Vacuum leaks. One repaired, no others visible.
5. Probably not harness. No visible cracking, though I haven't cut into the main line sheath.

Codes: I have had at various times 5, 6, 19, but they come on at stalling, so I'm not sure if they are the cause or the effect.

Since I suspect a mixture problem, I'm leaning towards the MAF, though presumably injectors or throttle bnit could maybe be suspect. I know zamar___ replaced his ECU when he had similar troubles, but I need to rule everything else out before parting with $1500.

I would like to validate the HFM air flow sensor before throwing $300 at it. Are there any good tests?

All ideas and tips appreciated.
Thanks,
Tom