Skype on the Mac is poor (or was) The call would degrade after a (inconsistent) period of time.

Mac networking has never ever been as good or as quick as Windows networking in my experience.
I use my PC laptop over the same wireless connection and Skype is fine, Mac version is not. Mind you I am using an earlyish external iSight not a built in mic which i am slightly suspicous of.

 

Try the beta it's as good as the PC now. Used to echo too much for me and quality was, as you say, erratic.