I've been well satisfied with Skype in the twice weekly calls we make from rural Australia to family in west coast USA - provided that we use our copper cabled ADSL2 system. Even the video is pretty well seamless when we choose to use it.
However any attempt to do the same using the mobile telephone system at our end - ie 'wireless internet' - whether by dedicated dongle or tethered phone - invariably results in disappointment and sometimes failure even in audio only calls - video is out of the question. In my experience the limiting factor may be the quality of your first step into the internet system - or a possibly reciprocal limiting factors at the other end.
Failing that, there may be other software recently installed which is slowing down the computer's ability to meet Skype's processing requirements.
That's not a solution I'm afraid - just an observation.