I had trouble keeping one of my Fords cooler; when I put new rubber seals around the radiator so that no air was passing around it and had to go through it,that solved my problem.Make sure the air is going through the radiator and there are no openings on the sides or bottom.
bit of both starts runnin warm when temp is around 91.5 to 104deg f thats when its up around 195 then i pull over and let it cool down
the higher temp one{195}will keep the coolant in the rad.longer to let it cool down more.if its hot and the 165 stays open then it wont give the fluid enough time in the raid.to bring the temp down.
I had the same with a s15 with a 327. fixed it simple. 1 15.00 bottle of water wetter,1 195 degree thermostat, 1 25.00 plastic 90 degree stock car fan. i could sit and idle in 85f and up and that ran 180-185 all day. same temp on the highway.
Up here there are 2 types of thremostats, 1 that the part that goes in the block is like a solid cup & works well & a cheap open bottom 1 that doesn't work all that well.