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Generally if the timing is jumping, I look for problems in the distributor drive, not the electronics. I am fairly sure your dizzy is driven off the cam. Check the cam drive (that is, timing belt or chain) for slop. If your ignition timing is off, everything else will be off--the injection will be at the wrong time compared to valve events, as will firing.
Alternatively a vacuum leak causing a stumble can screw up the engine speed enough to confuse the ICU. It has to calculate firing based on the engine speed; a stumble of a couple milliseconds will cause early/late firing which will make the timing appear to jump.
So I think that's an effect, not a cause.
Check for vacuum leaks before blowing money on the electronics. I am not an expert at all in the 240 but I've worked on a lot of electronically controlled cars and it sounds more like worn valvetrain components or a vacuum leak to me.
The fact that so many folks are suggesting replacement of the intake manifold gasket tells me that it should be one of your first tasks; while it's apart you have easy access to vacuum lines and other possible problem sources. I think the flame trap is down there too? Might be the ideal time to give it a good overhaul.
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