Facetime audio is broken-up

I am in Oregon using Comcast cable, Sister using 20" imac in Sacramento on AT&T. Facetime audio is broken up primarily from her to me. Apple says her computer mic is good but facetime from the apple store to me yielded same poor audio. Using Skype, same problem. Haven't tried YM Chat though.

My computer talks fine to Philippines using YM, records audio clips just fine. If she uses iPhone to facetime to me on my iMac, all is good. I am truly baffled by this. Looking for someone to facetime with to eliminate problem being on my end with Comcast... I think perhaps it is on her end with AT&T but need to eliminate my system as a possible source of problem.. Can anyone help in any way to solve this dilema?