MultiCraft memory at 100%

Hey, I own a minecraft bukkit server hosted by StreamLine MultiCraft. Lately my memory has been at 100%. I have no idea why it is doing this. My max memory is 512mb. The server keeps crashing due to this.

WARN at org.maxgamer.QuickShop.QuickShop.loadEcon(QuickShop.java:365)
11.01 16:59:03 [Server] WARN at java.lang.Class.newInstance(Unknown Source)
11.01 16:59:03 [Server] WARN at java.lang.Class.newInstance0(Unknown Source)
11.01 16:59:03 [Server] WARN at java.lang.reflect.Constructor.newInstance(Unknown Source)
11.01 16:59:03 [Server] WARN at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
11.01 16:59:03 [Server] WARN at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
11.01 16:59:03 [Server] WARN at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
11.01 16:59:03 [Server] WARN at org.maxgamer.QuickShop.Economy.Economy_Vault.<init>(Economy_Vault.java:8)
11.01 16:59:03 [Server] WARN at org.maxgamer.QuickShop.Economy.Economy_Vault.setupEconomy(Economy_Vault.java:16)
11.01 16:59:03 [Server] WARN java.lang.NoClassDefFoundError:


Please help!

11.01 17:11:50 [Server] WARN Can't keep up! Did the system time change, or is the server overloaded? Running 3865ms behind, skipping 77 tick(s)