Loose rivets,
The inherent problem with Skype is that its design is "peer-to-peer".
This means that Skype actually have very little real control over the quality of your conversation because they are fundamentally reliant on the resources (or lack thereof) made available to them by other Skype users.
So when you think about the sometimes vast quality difference in internet connections, computer spec and computer load you'll understand its pretty much a lottery as to whether the chain of computers you go through to reach your destination leave you with a decent transmission channel or not.