Intermittent Check Engine Light

[jgardner72]

1994 325i. 172,000.
New plugs, new O2 sensor, fairly new air filter.
After replacing O2 sensor, all was fine for a couple of months -- good mileage, etc.
Recently, the check engine light started to come on only when driving in the city. When revving the motor or driving on the highway, the light clears.
I have initiated the diagnostic program (step on the gas pedal 5x when key in position 2) and it only reports the O2 sensor.
What could be causing this???