05 F150 4x4 with the 4R75 transmission. When coasting for a few seconds at city speeds, if I quickly push the accelerator to 3/4 throttle, the engine free revs for about a second and then the transmission violently slams into gear. I have no other symptoms of transmission problems than this. This can't be normal. Fluid level and quality are fine. Any idea what I should troubleshoot first? 100K miles.

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