This is becoming a constant nightmare. First it clears up for months and then it returns. On and off, same thing. Its not even a continuous problem as it will completely go away on certain days. The car will idle rough, and have a slight stumble/hesitation on acceleration from standing. Here is the list of things that have all been done:
Spark plugs replaced today
Valve stem seals replaced last week
Oil/oil filter changed last week
Coolant flushed/changed last week
Was getting Error code 12 for EZL, replaced this week. code is now gone.
O2 sensor replaced 8/2014
Radiator replaced 7/2014
All radiator/coolant hoses replaced this year
Voltage regulator replaced 7/2014
Fuel filter replaced 8/2014
Fuel pumps replaced 7/2014
Distributor cap replaced 3/2014
Ignition rotor replace 2014
Spark plug wires replaced 1/2014
Throttle body gasket replaced last week.
Both IACV hoses replaced 3/2014
IACV tested ok 2015
O2 sensor tested ok 2015
Vacuum hoses replaced 2014
Alternator upgrade to 120 amp 6/2013.

Not receiving any check engine codes other than code 11 for secondary air injection (disconnected smog pump). I checked the duty cycle and the readings fluctuate rapidly between 45% - 55% per the ke-jetronic write up. There was a nice puddle of oil in the intake manifold after I removed the throttle body. Once the valve stem seals were replaced, all is dry now. Anyone got any suggestions?

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1990 190E 3.0L