If you did hear some crackling from that, then you'll need to get that lead replaced. It could be just the contacts of the plugs that have become a little tarnished. A good ink eraser with a fairly sharp edge should clean these up nicely. With a Rockwell Chipset OK, you've carried out all the above and you are still getting poor high speed connects with your modem. Now it's going to matter exactly what type of modem you have, and whether it has a Rockwell chipset in it or not.
Higher figures mean worse connections.
Anything higher than about -24 with the first, and you'll be seeing bad connects or possibly none at all. Higher than 15 on the second, and you've got real line problems on your circuit that your Telco should be able to sort out. Ideally, this figure should be 0. If it is 1 or 2 after you have been online for a while, then you probably have nothing to worry about. If it climbs much higher over, say, a 3-4 minute connect, then you are probably suffering from noise or low level or both, as in the Rockwell situation above.