2007 GMC Sierra Do I have a transmission problem

Yesterday, my 2007 Sierra seamed to slip into neutral on 3 separate occasions. All involved city driving at a speed no higher than 45mph. When the problem happened the speedometer would drop to zero, and the engine would rev like I shifted from drive to neutral. But I never shifted anything. In response to the problem I would shift into 3rd gear and back into drive, I also hit my towing/overdrive off button on the stearing column. The problem would then stop. I don't know if my shifting really helped.

Where should I look?

Thanks