Ups and Downs (comb-like) in Bandwidth Graphs

[raynet]

Dear Friends,

We are a running ISP using Cacti and STG to monitor our send and receive Internet bandwidth usage. I have attached the screenshot of my graph. The thin white spaces between the chart is our link being down, so we call it downtime. But, I want to know, in your opinion, do those comb-like or step-like ups and downs normal knowing that or is it related to our uplink being weak or our quality of service low?

Please advise.