I have the same issue the Ghps (not work share) is going down. I had 620Ghps mining here, 3 weeks later it's 530Ghps!
What kind of ASIC device(s) is this? What hashrate is your mining client showing?
Are you getting a lot of rejected work? Hardware errors? Connection problems?
to follow up on the doctor's questions.
looks like you have 3 s-1's is that the case?
Or if different gear is the working area hotter due to summer heat?