Newbie with stock tune: slowly increasing speed and RPM in 2nd gear w/o gas pedal

I am very new to this. Stock tune on 5.3L slowly increases RPM when in 2nd gear and not depressing the accelerator pedal. The truck shifts into 2nd gear about 12 mph when easing around the subdivision. In 2nd, about 1000 rpm, and without applying any throttle via the pedal, the rpm and speed slowly increases. I have to ride the brake to keep the speed below 20 mph so I'm not sure at what speed/rpm the truck would stabilize. Any ideas on what is causing this and more importantly how to make the "rolling idle" stable in 2nd gear?

Can you post your tune? What converter are you using? Have you changed or modified the throttle body? Have you adjusted the set screw on the side of the throttle body?

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Sounds like it's riding the spark advance table. Post your tune and log.