1998 Chevrolet Express 1500 Question: My transmision drops in and out of gear in hard excelleration from 2nd to 3rd.

I have 213,000 miles on my van and never had a problem and this dropping out of gear almost feels like the engine is hesitating but I know it is not that but is the trans. When I let off the excellerator it goes back in gear and I continue on ok.
Is there a trans computer that could cause this condition or do I need a trans rebuild. If I drive easy this does not happen but I am afraid it could stop working when I am out in the middle of nowhere.