Tranny problem

Truck is in the signature. Cable from gear selector to indicator is broken. Truck has done this twice now. When pulling away from stop sign gradually rolling on the throttle between 1st and 2nd gear shift, feels like the truck drops out of gear. I have to manually move the gear selector to neutral and then back to drive and then truck goes back in gear, and drives fine. Only does it every now and then like I said it has happened twice. Truck still pulls fine doesnt feel like slippage or anything, and if I am heavy on the skinny pedal it has not done it at all. It is almost like it just popped out of gear. But I am wondering what would cause this. Slack in the linkage or something worse? As always any helps is greatly appreciated.