Overheating problem..Weird..help

Hey guys i just got my car back from the shop i had alittle problem with my dss block. im not talking bout that now tho...but anyway..everything needs to be broken in again, and as i was driving home , in about 85* weather here...she would rise to about 205 and stay there steady, sometimes drop to 200...but as soon as i went up a mountain..it rose to 210 in no time, then, as soon as i went down the mountain, it dropped to like 190-195...how does that make sense? could someone explain to me, also it was bubbling out over the overflow tank. When i got home the tank was almost filled to the top, and if i let her idle the temp rises to 210 or alittle more if idleing longer....
what the hell?
maybe it need to be burped?

ok well listen to this...when i was driving tonigiht..and it was about 50* out. My car was sitting shut off for about 15 mins. i started it up, drove off...and my gauge shot up from 180* straight to the end of my autometer gauge...like 250*!! it just shot up...now there was no way the engine was that hot...it didnt even feel it when i opened the hood. Then it would slowly climb down and jump around again. Is my gauge or sender messed up? Or could my thermo be stuck? When i feel the upper rad hose, its very hard. u cant squeeze it?