Well, he reported that the tach signal immediately dropped to zero when the stalling happened. This can be explained by the crank sensor that fails to deliever the tach signal both to the ignition and the tachometer. I had this happen on my car: ignition stopped at 65 MPH and tach dropped to zero immediately. A failing MAS unit would not give such symptoms (at least not that I am aware of). Also there are several reports both on this forum (as well as on the German forum) that for some reasons it is a common pattern for the crank postion sensors to stop operating at high engine temperatures first.
Have a nice day, Bruno

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1992 300TE 160 kmiles