Hello,
Can you please provide a link to a page that is exhibiting the 500 error? I went to every page on your site and refreshed each of them a few times and did not run into the issue. I see your site is using plain HTML so the normal reasons this error would occur do not really apply to your site.
As for site speeds, the free hosting servers are just unfortunately not very fast. The servers themselves are pretty powerful, but the fact that there are thousands of accounts on it means that the resources are spread more thinly. There are a few things you can do however that would speed up your site:
1. Enable CloudFlare by logging into cPanel. Please note that if you do this, you will need to access your site by
www.yourdomain.com (with the www. in front), as the non-www will bypass CloudFlare due to the current setup we have with them. CloudFlare is a free service and you should notice a decent if not significant speedup using them because all of your pages have static content by nature of being pure HTML
2. Compress your CSS and javascript pages and combine them all into one file (well, one file for all your css and one file for all your javascript) if you have not yet done so. This will reduce load times because the server doesn't have to send all those spaces, tabs, and newlines so it shaves a decent amount of the output down, and it also only has to serve 2 extra files instead of however many you would otherwise have.
3. Try to reduce image file sizes as much as possible. You can probably get away with using indexed gifs for the site layout images (background, etc.), and use medium quality jpgs (50-75%) or pngs for the other images. This will only have a noticable impact on your site speed if you find the site itself loads fine, but the images on the site take forever to load.
4. Upgrade to Illuminated or Premium. These servers have far less users on them, which means more resources would be available for your site to use, which means in general that your site would load faster
Let me know if you have any more questions