A FOKKER 28-4000 IN CLB AT FL215 DECLARED AN EMER AND DIVERTED DUE TO UNCOMMANDED QUICK RUDDER YAW MOVEMENTS WITH #1 RUDDER UTILITY HYD LIGHT RAPIDLY ILLUMINATING CAUSED BY AN INTERMITTENT RUDDER PWR SHUTOFF VALVE AND AN IMPROPERLY DEFERRED RUDDER CHANNEL UTILITY LIGHT SYS.

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DURING TKOF ROLL AT ROTATE SPD; THE FLT CTL SYS RUDDER CAUTION LIGHT ILLUMINATED; ALONG WITH 'FLT CTL' MASTER CAUTION LIGHT. THE RUDDER LIGHT HAD PREVIOUSLY BEEN MED'ED AS INTERMITTENT. AFTER TKOF THE RUDDER LIGHT SWITCH WAS SELECTED OFF IN ORDER TO EXTINGUISH THE MASTER CAUTION LIGHT. PASSING THROUGH FL215 THE ACFT BEGAN TO YAW QUICKLY BACK AND FORTH. THE UTILITY SYS RUDDER LIGHT RAPIDLY ILLUMINATED AND EXTINGUISHED FOR A PERIOD OF 2-3 SECONDS. THE AUTOPLT WAS IMMEDIATELY DISCONNECTED WITH NO CHANGE. THE ACFT RETURNED TO NORMAL AFTER 2-3 SECONDS OF OSCILLATION. I ELECTED TO RETURN TO PDX. ENRTE BACK TO PDX; THE ABNORMAL CHKLIST FOR BOTH RUDDER LIGHT ON WAS ACCOMPLISHED. THIS RENDERED THE ACFT IN MANUAL REVERSION. THE LNDG WAS NORMAL. ACFT WAS DELIVERED TO MAINT FOR TROUBLESHOOTING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A FOKKER 28-4000 AND DEPARTED WITH A DEFERRED ITEM ON THE RUDDER HYD PWR UTILITY SYS LIGHT. THE RPTR SAID WHEN MAINT WORKED THE ACFT THE CORRECTIVE ACTION WAS FOUND TO BE THE #2 SYS INTERMITTENT RUDDER PWR CTL SHUTOFF VALVE WHICH IS NORMALLY DE-ENERGIZED OPEN. THE RPTR STATED THE DEFERRED ITEM FOR THE UTILITY CHANNEL LIGHT SYS SUPPLEMENTAL PROCS WERE CONFUSING AT BEST AND THE COMBINATION OF AN INTERMITTENT SHUTOFF VALVE IN THE #2 SYS CAUSED THE OSCILLATION OF THE 2 RUDDER PWR UNIT SHUTOFF VALVES. THE RPTR SAID THE COMPANY HAS REVISED AND AMENDED THE MEL SUPPLEMENTAL PROCS TO PRECLUDE ANOTHER INCIDENT. THE NTSB HAS CLASSIFIED THIS AS AN INCIDENT.