B757 FLT CREW EXPERIENCES ENGINE BLEED AND N3 RPM FLUCTUATIONS DURING CLIMB AT FL200. THE FLT CREW ELECTS TO DECLARE AN EMERGENCY AND RETURN TO DEPARTURE ARPT.

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DURING CLB TO CRUISE AT ABOUT 16000 FT; THE R CTR TANK FUEL LOW PRESSURE LIGHT CAME ON. WE PERFORMED THE CHKLIST ITEM AND CONTINUED. ABOUT 3 MINS LATER; THE R ENG N3 GAUGE BECAME ERRATIC AND INDICATED ABOUT 55%-85%. ABOUT A MIN LATER; THE R BLEED GAUGE INDICATED A PRESSURE SURGE FROM 10%-40%; WITH A CORRESPONDING PRESSURE SURGE FELT BY US AND BY MANY IN THE CABIN. THE R BLEED LIGHT THEN ILLUMINATED. WE PERFORMED THAT CHKLIST. MOMENTS LATER; WE RECEIVED A STATUS MESSAGE 'R ENG SPD CARD.' WE CONSULTED THE CHKLIST. WE WERE NOW AT 20000 FT. WE MADE AN IMMEDIATE DECISION TO RETURN TO ZZZ. I TOOK CTL OF THE ACFT WHILE THE CAPT NOTIFIED DISPATCH TO INFORM THEM OF THE PROB AND OUR INTENT TO DECLARE AN EMER DUE TO THE MULTIPLE SYS PROBS AND THE OVERWT LNDG. THEY AGREED. THE CAPT TOOK BACK CTL OF THE ACFT AND WE IMMEDIATELY DECLARED AN EMER. WE WERE CLRED BACK TO ZZZ BY ZZZ CTR AND THEN HANDED OFF TO ZZZ TRACON AND ZZZ TWR. WE PRE-BRIEFED THE SINGLE ENG LNDG PROC AS A PRECAUTIONARY MEASURE. WE LANDED WITHOUT INCIDENT AT LESS THAN 100 FPM DSCNT AND SMOOTH TOUCHDOWN.