Loading and Access problem (Wordpress)

Hello,
This is the 4th time this problem is happening to my site, in 1 month. 1) Time to time, it's become very difficult to load the site. Its can take 1 or 2 minute to see the home page (www.positifstyle.com). Same thing on smart phones... What ca I do to make my website run faster?

2) Time to time, the website become unavailable. You can't see it and worst, I can't even log in my Dashboard. This morning, the website is available, but I can't login...

I was thinking that this problem come from my web hoster, I contacted them, and they say that everything is fine from they part. What can I do?

Thank you guys for your help. Learning Wordpress is exiting and fustrating in the same time. Have a good day.