slownis

I took the family on a 4 hour trip today in our 2002 Chevy Suburban LT 2wd. The symptom is not always there. We only notice it from time to time on long trips. What happens is, we will be cruising along the highway and as we come to a slight uphill section of road, after giving just a tad more gas 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 its not always on the highway either. It seems to become more prevalent after an hour or two of freeway driving.

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

Thanks!