For PrestaShop v1.4 through v1.5.2

Open config/config.inc.php
On or around line 29 you will find this line
@ini_set('display_errors', 'off');
Change that line to read
@ini_set('display_errors', 'on');

For PrestaShop v1.5.3+

Open config/defines.inc.php
On or around line 28 you will find this line
define('_PS_MODE_DEV_', false);
Change that line to read
define('_PS_MODE_DEV_', true);

or the problem may be of you CPU Usage, its very less and you are on shared server, so when visitors come in larg amount to your site its give the problem, it is better to move your site on your own VPS instead of shared server

Yes, if your site goes down again and again for long durations you will start loosing rankings. Get this fixed immediately.

Check google webmasters regularly to keep a check on google crawl issues.
And use some free service like uptimemonitor or pingdom monitor to get alerted the moment your site gets down

Does this happen when you're just browsing the blog? If it only affects the admin panel then it should not have any effect on your search engine rankings.

I'd still switch to a better host.

It's not that simple. Shared server tends to have more resources available even when they're limited. If OP is not getting a beefy VPS or optimizing the LAMP stack the performance may actually be inferior to that of a well-managed shared hosting server. In addition a VPS requires either Linux admin experience or managed account (probably +$xx to the price).

It's much easier to switch to a company that isn't overselling their shared servers to the max, and has fever users per server.

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