Steam download slower than single-channel ISDN despite 50,000 VDSL (long-distance gamer than ISDN/telephone modem)?

I've been wanting to download SFM via Steam for days, but I get 0 KB every 2 seconds for several seconds up to minutes, which always starts at 5 GB, and when it downloads it's at modem speed, so way under 64 KBit/s, then it's 0 KB for 2 seconds up to 2 minutes, then…