[W126 Coupe] Running Hot update

[Pavel]

Here is the update on the sec crew.  Put a brand new behr radiator in
pulled drain plugs from block drained all the coolant filled it up with MB
50/50 antifreeze mix.  Started it up got all the air out of the system, the
stupid thing still sits at 90 unless I rev it up to 2000 rmp then goes back
to 82 deg instantly, then creeps up to 90.  Stays between 88 90 while
driving around unless you hammer on it than goes down to 85.  It bugs me
very much and it is not enjoyable for me to drive my sec anymore.  Is this
getting caused by my new wahler thermostat?  Why did they do that?  Where
do I get the original thermostat so my temp gauge does not drive me nuts.
thanks
Pavel