Why is my 1998 E300TD engine overheating?

  mercedes/e-class/1996-2003

Started running hotter than normal, power slowed then recovered, heard a little klunking like sound but stopped, from the a/c (which kept working fine!) then after a couple rides started overheating, changed thermostat and still overheats! Cools some when going slow but increases in heat with speed! The temp gauge bounces slightly too!

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