Some have mentioned issues that appear to point to Alter.net routers and the datacenter itself where some servers are located for example.
Finding the actual cause is not simple in most cases.
Edit - If you are on wireless (bane of lag spikes) , try connecting to your router directly to eliminate this as a cause.
I can't answer for others but my computer is hard lined into the router. And it started at like 1 in the afternoon for me on Tuesday so it wasn't exactly a peak time either. Hell the only reason I was even home was because I was on two days off from work cause I busted up my knee on Monday at work. Which really sucked because I couldn't my HM dailys knocked out.