B737-800 LEFT ENG EXHIBITED HIGH VIBRATIONS DURING FLT. AFTER THE SUBSEQUENT TKOF; THE ENG EXCEEDED EGT LIMITS AND THE FLT RETURNED TO THE DEP ARPT.

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ACFT HAD SEVERAL RPTS OF HIGH VIBRATIONS ON BOTH; BUT HIGHER ON L; ENGS. FLT ATTENDANTS RPTED ON PREVIOUS TURN AROUND FLT THAT AIRFRAME VIBRATIONS COULD BE FELT THROUGHOUT ACFT. WE DID A DERATED TKOF WITH NORMAL TEMP INDICATIONS BUT VIBRATIONS INDICATING SAME AS DOCUMENTED BEFORE; 2.4L/1.7R; DURING CLBOUT. DURING OUR DEP FROM ZZZ THE WINDS HAD BEEN RPTED AS A TAILWIND SO WE WERE SET UP FOR A 26K MAX PWR TKOF; DUE TO WT 26000 LBS. WE GOT A CURRENT WIND RPT DURING TAXI OUT AND WIND HAD GONE TO CALM SO PUT IN THE DERATE FOR THE 26K TKOF. TKOF/ENG INDICATIONS WERE IN THE NORMAL RANGE FOR TKOF. DURING CLBOUT WITH CLB PWR SELECTED APPROX 7000 FT THE #1 ENG EGT BEGAN TO FLUCTUATE IN THE CAUTION THEN RED RANGE. I SAW IT FIRST BECAUSE IT OCCURRED DURING A RADIO CALL WITH SOME TURN/CLB/DIRECT ROUTING FROM ATC. I DREW THE FO'S ATTN TO IT AND THE EGT SUBSIDED BACK OUT OF THE HIGH RANGE. IT THEN PEAKED AGAIN IN THOSE RANGES AND SUBSIDED ONE MORE TIME. WITH BOTH OF US WATCHING IT CLBED INTO THE RED RANGE AND STAYED THERE. I STOPPED THE CLB AT 10000 FT. WE RAN THE ABNORMAL CHKLIST FOR ENG LIMIT EXCEEDED AND RETARDED THE THROTTLE OUT OF THE RED RANGE. AFTER DECLARING AN EMER WITH ATC AND BRIEFING THE CABIN CREW I MADE AN ANNOUNCEMENT TO THE PAX THAT WE HAD DEVELOPED A PROB AND WERE GOING TO RETURN TO ZZZ. WE DID THE CALCULATIONS FOR THE OVERWT RETURN; 154600 LBS; AND MADE A NORMAL RETURN.