Sticky piston randomly leave's blocks (same contraption works in sp)

http://i.imgur.com/XbUUBFh.png Pistons are activated by comparator that gets activated when an item passes trough a hopper.

Plugins: ColoredSigns, OpenInvv, CraftIRC, WorldEdit, HealthBar, Modifyworld, PermissionsEx, LogBlock, WorldGuard, PermissionsExTabColors, dynmap, ChatEx