I agree; I’m having the same wave of issues. It goes anywhere from utterly CRASHING to totally working at optimum speed!! I was told because it’s a shared server, not DEDICATED, that its likely to waver.
But just like you, my site had been fine before then one day kaputz. I’m sure my issue was due to a spike in traffic, but still. I dont think it has to do with shared vs dedicated, and not worth upgrading to dedicated unless your site is a REAL top name site.
So I ran a diagnostic of my site with https://websiteoptimization.com and found that essentially I “had too much crap” on my site. I’ve since then upgraded from economy to Deluxe with GoDaddy, being told that would help the speed. It seems to help right now but it could be coincidence…