Temperature gauge in 95 s10 blazer isnt reading accurately. How to fix?

Randomly, my temp gauge started acting up and not rising up past the 1/4 mark (or around 150 degree mark.) Every once in a while , it raises up to 210 degrees, but only for a second the it drops down again. I tried replacing the old therostat with a new one and I replaced the coolant sending unit. Nothings working. Coolant also disappears from the resovoir after i top it off a couple days before. Any ideas of what this problem is?