How many miles? Sounds like the input clutch piston issue, that will require a full rebuild to fix. It usually starts off as feeling a little bump on takeoff, like it wasn't fully engaged at first. Then over time it gets worse as it seems it's completely in neutral til you give it gas. This seems to be a common thing lately. Every time I see it, it's on a 03+ 4t65e or an impala of 00-05, although I don't know of any changes in that particular area of the transmission.
You can try adding like half a bottle of lucas transmission stop slip, thickens up the fluid a bit (and does things to the seals) and should lower the amount of leakdown on the input clutch piston sealing area. Eventually it will get worse unfortunately.
Does the way I described it sound like your case? If it's just a clunk noise and that's it, check your motor and transmission mounts as they might be pretty loose. Have someone shift from reverse to drive and watch the engine for excessive movement.
