Reasons for skippy gameplay

So, since 1.7 the game runs skippy in most places (but I recall a sim or two where Bandwith bar bounces merrily around 300-400 kbps and everything's smooth) and I would like to know what is the reason for it. Is it:
a) crappy connection - I have ADSL, 640 kbps downstream, 128 kbps upstream.
b) not enough disk space - I have cache set at 500 megs and free 3,5 gigs on my HDD.
c) mess on the server - due to the update (or the g-ball incident) some things over at Linden's aren't exactly as they should be. Some of the patches partially solved the problem (getting decent bandwidth in some places - not all, but some "empty" sims load in a flash and Welcome Area runs quite well).