Bad earth troubleshooting

My engine has been a bit "moody" when idling. It can idle smooth, and then suddenly change idling speed. Some of the dash gauges has also been flickering a bit. It haven't bothered me much but today I took my USB oscilloscope and started troubleshooting as I figured it must be electrical. My suspicions were the alternator as it use to haunt me.

But I was wrong. Here is the voltage between the battery negative and the engine loom ground:

I moved the loom's ground connector from a pedal box cover screw, to the engine block. Now it look like this:

The scales are different but you get the point. One could argue that bumps around 50mV won't do much harm but at least now the oil pressure gauge is more stable. It feels like the engine runs smoother too, but that might just be placebo.