2002 Suburban Transmission occasionally slips on the highway during ro

[slownis]

I took the family on a 4 hour trip today in our 2002 Chevy Suburban LT 2wd and noticed some 'slipping' when the automatic transmission was trying to downshift from 4th to 3rd. The symptom is not always there. We only notice it from time to time on long trips. We will be cruising along at highway speed and as we come to a slight uphill section of road, after giving just a tad more gas to maintain speed, the engine will rev up pretty high and roar, but not much acceleration. I've also noticed this occasionally with the cruise control engaged when going uphill - we'll hit a slightly uphill section of road and the engine will start to roar unexpectedly. The problem is never noticed when driving around town, and it doesn't always do it on the highway either. Theres a 55mph stretch of road between the kid's school and home and I tried to reproduce the symptom on the way home unsuccessfully. It seems to become more prevalent after an hour or two of freeway driving.

 

The truck has 180k miles on the clock, but is otherwise in decent shape. The person I bought it from a year ago had owned it less than a year, and she said the previous owner had replaced the transmission.

 

What is this a symptom of and is there any steps I should take before this gets worse?

 

Thanks!