My 79 Chevy has rough idle/miss at idle but runs fine otherwise

It sounds like you have a vacuum leak or a carburetor problem. Check all hoses and the line going to the brake booster for cracks or leaks. Make sure the carb bolts are tight at the manifold. You can check for leaks using some carb cleaner. Spray around the base of carb while the engine is running, any leak will cause the engine to sputter when the spray hits it. You may want to look into rebuilding the carb or replace it. If its the original its a Quadrajet, they are not the easiest to rebuild but not impossible. If you rebuild it yourself, make sure to get the numbers off the carb so you can get the correct rebuild kit and accelerator pump. Plus you should replace the float when you rebuild since its 30 years old. A thorough cleaning of the carb is crucial before rebuilding. The fuel mixture screws have caps on them so they wont be altered after assembly from the factory. You'll have to remove those caps to remove these screws before cleaning. You can probably find some good info. on doing this at a good parts store. I hope this helps you.