Tranny acting up in 99 3500

[TheMonacoans]

After coming down a long hill in high gear when I step on throttle the tranny doesn't engage. The engine just revs up, like it's in neutral. I have to drop it into 2nd and then it engages and I can shift back into drive and it's good again. This doesn't happen on every hill but it did happen a couple times yesterday on a 6 hour drive. Anyone got an idea what would cause this?

Bob

 

[twocoda]

You may want to check your tranny fluid level and inspect for possible vacuum leak