I believe what happens is the phototransistor slew rate slows to the point it basically runs into saturation (on all the time) when it gets hot. Theres an "equivalent" sensor that was used by Chrysler in a traditional distributor application that was used heavily in cheap aftermarket optis that did this. The Chrysler application was designed to work in a thermosplastic distributor body, and wasn't able to handle the heat of operating while attached to an aluminum plate bolted to the front of the engine block.
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