99 5.3L Idling Problem

I am having an issue with my '99 Sierra with the 5.3L (T) motor.
On a cold start the engine idles fine. After it warms up(15-20 Min) it will stall or nearly stall at idle running very rough at idle speed but running
fine past 1000 RPM. This goes on for about 20 minutes then the problem goes away again.
The truck has ~95K miles, regular oil and air filter changes but no other maintenance beyond that? What is a likely cause of this problem? I was thinking of changing the spark plugs.
Thanks in Advance, -a12vman
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