What was wrong with the free float tube?
When trouble shooting an AR I always start at the gas port and work my way back. Make sure the ports in the gas block and barrel are aligned. Even though I've never seen a problem with the gas tube or key I check them for blockage and eliminate them. Then I check the charging handle to make sure it's not bent, the gas key rides a long the top of it when cycling and if it's bent it will cause the bolt to short cycle. If the charging handle is straight I go to the bolt and bolt carrier cleaning it and change the gas rings. Then make sure you lube the bolt and BCG with a good quality lube (I personally like Tetra gun grease). I doubt seriously that there is a problem with the buffer spring unless it was replaced with an extra power spring at some point, but I have seen bent buffer tubes that would cause issues.