SOURCE: my 1991 Buick Park Avenue
there is a little magnet inset in the camshaft gear that the camshaft sensor gets its signal from. this magnet, over a period of time, gets loose and falls out. the reason for the smooth running when the light is on is the ECM sets the ignition timing to base due to the loss of camshaft signal. to replace the magnet you will have to remove the timing cover to access the cam gear.
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