Hello! Simply put, you are having "bugs" because you used different position properties in your CSS; (i.e. #_head{position:fixed}, #_menumobile{position:absolute}
because of your CSS, the containers in your page are being displayed inaccurately;
+ you cannot make a website look the same in all resolutions;
+ my suggestion is just rewrite your HTML and CSS, preferably using some framework (Materialize, Bootstrap etc.)
+ your HTML is not SEO friendy, and in most tags you have inside styling (i.e width=100%)
For those reasons you are having serious troubles!
If you need help, feel free to contact me!