My keyword dance heavily, any reason ?

Hi Folks,

I have a keywords dancing like a crazy, for example 3 weeks ago it was #22 then it disappeared for "2 weeks" from the serp, then just 2 days ago it appeared again at #11 then it disappeared again yesterday so it's nowhere in serp again !

Note:
-This KW is pretty old and competitive also
-I'm hosting with Hostwinds (shared hosting 6.5$/monthly)