Minecraft was built using Java 1.6 to 1.7. Upgrading to Java 8 will mean that some methods of previous versions are deprecated or have changed in the current version until the game is fixed/updated to the current version of java.
Best to leave it is until it's supported. *Java 1.7*
Besides from this, Can we see the FULL server.log, Bukkit/Spigot Versions and plugins list. These should already be in your original post any ways.
You should also try deleting java COMPLETELY and installing JDK As some multi-threaded .jars need JDK.