I think you really do have a ground point at the driverside mount, you just are not seeing it. A few years ago I had to replace the head gasket because of a large oil leak. After reassembly the car, it started right up, I let it warm up and checked for coolant leaks, turned it off and went to wash up and go for test drive. Thirty minutes later I returned for the test drive and the car did not start. I had miss the ground tang to a wire that holds the harness down as it passes over the engine mount bracket. There are 3 or 4 10mm bolts that hold the harness down, the wire goes to one bolt that has a nut on the end. I guess while it was hanging loose it was grounding by accident, then as I went to start it again for the test drive, I assume the shaking of the engine at shut down or start up caused the wire to move and no longer be grounded. But that ground point grounds the 02 sensor and the power stages for the spark plugs. Your codes of 113-115 are for the injector groups, I don't think it is coinincidence that you are having this problem. There must be another point that is missing. I had that code once too and am not sure how I cured it, but seem to think it was corrosion in the big plug connector attached to the driver side wheel well. That is easy and quick to rule out.

DanR 94 964 241,000 miles
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DanR