2003 Oldsmobile Silhouette engine miss

Have a 2003 Oldsmobile Silhouette 3.4L that has recently started to misfire badly. I know the design of these engines is susceptible to head gasket leaks, but the temp remains well within the normal range and it does not appear to be losing coolant as you would expect with a leaky head gasket. With the trans in P or N, the engine will idle roughly, but if you give it enough gas to increase rpm to 1500 & above, it smoothes out & appears to run just fine. In gear, however, the miss is almost always present. It will intermittently disappear if the engine isn't under too much load, but for the most part, a misfire in 1 cylinder (sometimes 2) is always obvious. The codes I was able to retrieve are: O2 sensor (2X), Bank 2 Lean, Fuel System Pressure, and Random Multiple Misfire (2X). Then the same code sequence repeats. I have checked the coil packs, wires & plugs. All check out ok. The underside of the oil fill cap will occasionally have a very slight tinge of evidence of coolant mixing, but the oil on the dipstick looks normal. Not what you would expect to see if there were a head gasket leak large enough to cause a bad misfire, and again, the coolant level has not dropped, and the exhaust doesn't have the "white smoke" appearance usually seen with a blown head gasket. Am I too easily dismissing a possible head gasket o problem? Could a bad O2 sensor cause that kind of misfiring? Or where should I look next?