Suggest using a factory or at least OEM replacement thermostat.
Replaced my less than one-year-old t-stat with an OEM replacement. Much better performance! Solved most of the hesitation problem and car runs far smoother. Gauge rarely goes over 87c degrees. (Make certain you buy the correct temp. unit.)
Search on thermostat replacement on this forum. Good procedure for my 1990 300E. Might work for you too. It took me 15 minutes and very little loss of coolant.
PS: Gas mileage went way up as well.