Try resetting the PRAM and SMC.
Restart the computer holding down the command - option/alt - P - R keys before the grey screen appears. Continue to hold until you hear the startup chime for the second time.
Try an SMC reset. The article covers when this is required, but doing this will not affect anything.
If the problem is still there, try booting into the Safe Mode. Shut down the computer and then power it back up. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart. For each disable/delete, you will need to restart if you don't do them all at once.