190e 2.6l o2 light / rough running problem

This car is a friend's and I've helped with it extensively. Symptom: The car will buck / cut in and out and flash the o2 light Cause: unknown - happens intermittently - car has done this for *years* but
recently it happens more regularly.
Attempted fixes: - new ignition components (wires, plugs, cap and rotor) - vacuum hose fixes - general clean-out and checkup of fuel distributor - new OVP (OLP?) relay
Historical fixes: - new o2 sensor - new fuel pressure regulator
It doesn't feel like a fuel issue - even bucking and kicking the car has decent power and will increase in RPMs during an 'episode'. It feels like ignition; like it's missing on a cylinder or two or three.
Any guesses?
I'm going to check wiring and fuses as best as I can to be sure there isn't a short somewhere or a loose fuse causing the computer or other electrical controls to malfunction.
I haven't been able to reproduce the problem in-shop so it's hard to know if it is actually not firing on some cylinders.
Thanks in advance for your help!
Damian
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I'd check:
1. the wire to the O2 sensor for being broken inside its insulation
2. fuel pump pressure and output volume
3. the crankshaft position sensor - if this engine has one
4. otherwise, the ECU - engine control unit - and its connections
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This sounds alot like the problem i'm haveing with my 91 300e 3.0 hard to start runs rough, stalls, acellerates ok after getting above approx 1500rpm will get to 7000 rpm in a hurry and shift. i replaced all the ignition parts also. I have done some diagnostics the coil and position sensor are good. Have you resolved your problem yet? Victor

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