2002 transmission drop at cruising

Hello all. Our 2002 when (and only when) cruising at around 70 mph has a brief (1/2 second?) drop of some sort in the transmission. It's just as if you very quickly turned the key off and on but I know it's not secondary ignition (my first thought) because the tach doesn't even wiggle. Anyone else? Is this the start of another transmission about to crap out? Only 28000 miles. Thanks.