intermittent 502 errors on sites

Hi all.

So We keep getting random 502 errors on all our sites that we host using plesk 12.5
This happens multiple times throughout the day and normally lasts for about 10 - 15 mins at a time and then just starts working again.

Please can anyone advise me on what to do about it?
This is what i have done last time it went down:

ps -efww
and this is the output:
https://www.surgemarketingsolutions.co.uk/temp/ssh_output2.txt

I then tried this to make it come back online using the...

intermittent 502 errors on sites