I have a 2000 5.9L automatic transmission Ram truck. I had it rebuild by the dealer, here's my issue. Sometimes when i hit the accelerator from a standstill, it feels like I'm riding on the clutch, then I pop it. So no power until 3 to 4 thousand rpm. It might happen twice a week.
My next issue is it doesn't feel like power is being transferred correctly to the rear wheels. I use to hear engine now it sounds like I hear my transmission.
Other than that the truck shifts fine.

 

sounds like you need to visit the dealer and have them look at it. it sounds like it's slipping in 1st.

 

I agree with steve00. sounds like a valve sticking or an apply piston. 3-4 thousand is a bit high for a take off rpm. Get it checked out and post back with your findings