I have exactly the same problem. Had this yesterday and on friday aswell, exactly at the same time. Site starts to load slow, gets slower and slower, and after like 30 minutes everything's back to normal. This repeats several times until the problem completely diasappears until the next day at the same time.
Support already told me the servers my site uses all work properly, it has to be my site... Just strange that all the other sites which are on the same IP as mine are exactly as slow