can a virus explain slow and erratic internet speeds over wifi?

Unlikely as at the moment there are no viruses 'in the wild' that effect OS X. There is adware, but I doubt that would be the cause of iffy wifi, there's likely something else causing the problem.

Here are some general trouble-shooting suggestions for wifi problems. I note that you are running OS X Lion, but many of the 'help' tips should still be valid:

http://osxdaily.com/2012/11/30/resolving-stubborn-wi-fi-connection-problems-in-m ac-os-x/

http://osxdaily.com/2015/01/30/os-x-10-10-2-yosemite-wi-fi-problems/

http://osxdaily.com/2014/10/25/fix-wi-fi-problems-os-x-yosemite/

http://osxdaily.com/2015/10/16/fix-wi-fi-problems-mac-os-x-el-capitan/