Despite the check engine light not coming on, I'd try to get any pending codes read.
In the absence of any codes, then the camshaft or crankshaft sensor would be a good place to start, in my opinion. The 2 sensors are the same, I would switch them to see if the behavior changes, but it seems like the crankshaft one is going out so replacing it wouldn't be exactly irresponsible. Preventive maintenance, if you may.
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