there probably having swimbladder issues.Goldfish are genetically prone to swimbladder problems because their bodies are so compacted.
If you feed flakes then they are gulping air at the surface and the air throws off their bouyancy. If you feed flakes you need to hold them below the surface of the water and let them sink.
If you feed sinking pellets then they prevent your fish from gulping air at the surface. The pellets themselves are full of air though. If you feed pellets you need to soak them in water for two minutes before feeding.
Another diet problem is that your fish food is high in protein. Goldfish should eat a lot of vegetation. They become constipated on such high protein diets. You should supplement their diet 3 days a week with shelled peas. You can also put a lettuce clip with romaine but they will prefer the peas.
It really hepls to starve your fish for 3 days before you change the diet. This will clear out the digestive system. It sounds like your problems aren't that bad though, it may not be neccesary.
I have an extremely sensitive Fantail who is a varocious eater. After eating he is always a little floaty but it passes after the pressure from his stomach relieves his swim bladder.
If the change in diet doesn't help he may have a minor bacterial infection. They sell medicated food that helps.