1994 E320 with 55k miles. The only thing I am certain has been replaced is the engine harness (part thanks to a kind member on this forum).

I noticed that the car drives different between idle-3500 and 4000+ RPM. At the lower range, it's a bit sluggish (but still quite driveable), but then once I downshift into 4000+ or rev it past there, it's like the car comes uncorked and takes off. Since the car is new to me, I'm not sure if it's just the uneven power band of a straight 6 or if there's an underlying problem. To my knowledge, the MAF hasn't been cleaned, it's on the original O2 sensor, plugs, wires, coils, etc. Oh, and sometimes at idle it hunts a bit, especially when cold; it's not rock solid like a Mercedes should be.

Recently, the CEL came on and I got two blinks=heated O2 sensor inop. I reset the light and it hasn't come back on since.

MPG in New Jersey in this weather is about 19 with mixed driving--a little low?

Worth changing out the O2 sensor just to be sure? Does this sound like O2 sensor symptoms?