97 jeep cherokee temp sits at 210 but is now going to hot then back down any help

first off is that the only coolant you put in it since you got it if so the water that you put in is gone from evaporation normally 50/50 you dont mix at all so i'd start with that add more 50/50 if problem still persists recheck the thermostat even though it's new it could still stick or be faulty i wouldn't worrie about the overflow bubbling mine does that too it wont hurt anything

If your coolant is bubbling in your coolant reservoir...you may have a cracked head. Don't take my word for it. My 97' Mercury Mountaineer would run hot and bubble, so I repaired it with "Blue Devil". Now my 97' Jeep Cherokee is doing the same thing.

GUAGE sender at rear of cylinder head- a pocket of air is common at this point- this allows sender to give highly innacurate readings- remove sender from head (take extreme care with sender they are in awkward spot) -then dribble coolant into sender hole till it overflows - then (after replacing thread tape on sender thread) replace sender in head- NOTE Engine slopes toward rear and allows an air pocket to be trapped right where sender is located!