A B757 FLYING AT 39000 FT IN ZAB AIRSPACE; SUFFERS ENG ROLLBACK AND SLOWLY DSNDS TO 28000 FT BEFORE THE ENG RECOVERS TO NORMAL PWR.

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AT CRUISE ALT THE FO AND I NOTICED (AFTER 3 HRS AT CRUISE) THE N1; N2; N3 OF #1 ENG WERE AT MINIMAL PERFORMANCE. FUEL FLOW WAS 2100 LBS AND THE EPR OF THE #1 ENG WAS EXCEEDING PARAMETERS AT 2.03. AIRSPD WAS DECREASING; #2 ENG WAS AT MAX THRUST (1.76 EPR LIMITED). I IMMEDIATELY PUT ANTI-ICE ON AND REQUESTED A DSCNT FROM CTR. I HAD THE FO CONTACT PHX MAINT; RPTED THE PROB AND RPTED THE PARAMETERS OF BOTH ENGS. AS WE LEVELED OFF AT FL280 THE #1 ENG RETURNED TO NORMAL AND NO FURTHER PROBS AROSE FOR THE DURATION OF THE FLT. I MADE A LOGBOOK ENTRY AND LABELED IT 'INFO ONLY.' MAINT IN LAS MET THE ACFT AND I FURTHER ELABORATED. I FOUND OUT LATER THAT THE SAME ACFT; SAME ENG HAD ROLLED BACK IN FLT AT FL390 JUST A FEW DAYS PRIOR. THE SAT AT THE TIME OF THE OCCURRENCE WAS -52 SAT; WELL BELOW ROLLS ROYCE'S RECOMMENDED TEMP FOR APPLICATION OF ANTI-ICE (-40 DEGS SAT IN CRUISE). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE ROLLBACK WAS VERY SLOW. HE ALSO DESCRIBED IT AS INSIDIOUS; ONLY WHEN AIRSPD STARTED TO DECREASE DID THEY NOTICE THE LOWER N1; N2; AND N3 INDICATIONS. ENG OF THE B757 IS ROLLS ROYCE 211-535E4. RPTR WAS NOT CERTAIN THE 535E4 WAS THE EXACT VERSION NUMBER. THIS ENG WAS WORKED ON BY MAINT BUT NO INFO WAS AVAILABLE TO SEE IF THE PROB WAS SOLVED. MAINT BELIEVES THE ROLLBACK WAS CAUSED BY ICING GETTING INTO ONE ENG PROBE. ALTHOUGH THE ENG ANTI-ICE DOES NOT HAVE TO BE APPLIED AT TEMPS BELOW -40 DEGS C; ICING CAN OCCUR.