What version of the IDE are you using ?
1.6.5 and 1.6.9 are still the best versions.
Also seems that some of the board managers versions are also an issue so what version of the board manager ?
Screen shots of your IDE with board / port selection may also be useful.
Some common causes are also bad or poor USB cables.
USB 3 ports
Some USB hubs (more so un-powered ones)