Rankings disappearing from Google and Server error (5xx) on GD files

Hi
I am not sure if you can help with this or if it is a GD problem.
My issue is that all was fine with my website until around 2 weeks ago when I noticed that certain pages were not appearing in Google any more. On checking my search console I had over 200 pages (out of approx 650) with errors on. I tried resubmitting them again individually and whilst some were accepted others showed Url is not on Google: Indexing Errors, despite the pages previously being indexed. Others showed URL is not available to Google: It cannot be indexed, but once again these pages had previously been indexed. I then noticed that a number of pages were showing Content wider than screen – Text too small to read – Clickable elements too close together and a number showed “Your page is not mobile friendly” Again all these pages had previously ranked and showed non of these errors.

My hosting company said “this can be related to the old Sitemap used by your website, to deny rules in .htaccess, to malformed/old/broken links” I dont know what they mean by old sitemap as I havent changed any? However they created me a new .htaccess file with the default code and today by errors have gone from over 200 down to just 4.

However the error pages are related to my GD files: gd-details/ gd-events/ gd-archive-item/ and gd-archive/ I get Server error (5xx) I have requested a validate fix again but not sure what the problem could be?

Any help or advice would be much appreciate as I am losing rankings fast and it is affecting my business.

Thanks in advance!