Looks like a capacity issue at Plus.net
Looking at old yahoo.com pages via Plus.net and an Brand X isp, Plus.net delivers frequent Wayback Machine "Failed Connection" errors, whereas Brand X delivers pages without a problem.
The rate of Failed Connections seems to be about 1 page in 3. Going to another page, then trying again often gets the page delivered. However, the response time is also much longer than Brand X.
Strictly, Plus.net aren't actually banning the Wayback Machine, but the Plus.net filter's lack of capacity is making Wayback unusable for legitimate purposes.