test your viscous fan clutch using the broomstick method, also check the electrix aux fan(s). If those check out, look for oil sludge in your coolant reservoir and/or an oil leak towards the rear passenger side of the cylinder head where it meets the block.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280