1979 G25 350

I tried this in the wrong place I think. I need so help, this is my moms van. She got it awhile back. Since then she has had the carb rebuilt, put in new spark plugs, new fuel pump and filter, and new air filter. But she is having a problem that when she goes on highway after 15 mins, it hesitates when she lets of gas for a bit its fine. Is there some thing else she needs to do to get this to stop? Any one know please let me know and thank you for your help.

If its a '79, it could be a lot of things, a vacuum leak on the distributor advance or the carb, a compression problem, ignition module, or it may just need a new timing chain.

When you say it hesitates, can you clarify?
Does it act like the key was shut off, (total deceleration), or just kind of "surge", hesitating monetarily or on-and-off?
I had a 1988 truck that would shut down after about 30-45 min on highway, and after a 10 minute cool-down with hood open, it would be fine again.
Got progressively worse, turned out to be intermittent shorting in the distributor's lower magnetic pickup area. (The shorts were caused by little chunks of that hard, white stuff that form in a distributor cap that doesn't get replaced often enough).
Good luck,
-Jim.

would have to agree bad pick up plate in dist.