Sounds like an intake leak. When cold, the choke is over richening the mixture and can cover up a vacuum leak. At higher speeds vacuum leaks can go un-noticed.
On the older Pontiac intake gaskets, they used a steel center with paper on both sides. I"ve changed lots of them in my day. The paper sides shrink down and get sucked into the manifold, leaving a small intake leak on each affected runner. Try spraying some carb cleaner where each intake runner meets the heads and see if you find where its leaking. Could also be a vacuum hose or carb base gasket too.