You see my KBits? It's at 5,100. You said yours is at 600 regularly. My DSL is at 1,500 regularly, and my DSL service is known for being slow.
If you say yours is at 250 KBits now, that's rougly 30KBytes. In other words, you download at 30 kilobytes per second, which is slow as HELL for DSL.
My only assumption is that you have bits and bytes backwards. It would make perfect sense if you're talking about bytes, and your problem could be diagnosed most likely. 600 KBYTES is a pretty average DSL speed if not above average, but 600 KBITS is an insanely below average speed. It's possible to get two linked 56k modems to download at 600 KBITS.
Like I said before, post what you got in KBITS and KBYTES, then we can at least see what the problem is.
-Lasereth