In my area AT&T has been pretty good with the installation advice, if you can get past the sales people and get to the "service and installation" folk.

That said, I've gone through three modems in about a year and a half of DSL use. In our old house, there was quite a lot of "user installed" inside wiring, and we had considerable trouble with phone lines run across the floor under carpet. (Two jacks, on two separate lines "disconnected" spontaneously when the carpet was vacuumed.)

The common indoor phone wire is two twisted pairs. Only one pair is needed per phone connection, and often if a line goes bad a "fixer" will swap wires. IIRC, the black and yellow are twisted together, and the green and red are twisted together. If someone uses a green/black, green/yellow, red/black, or red/yellow "pair" you're almost guaranteed to get crosstalk - which may be tolerable on the voice lines but could interfere pretty seriously with DSL.

Your DSL connection should be directly through a "master" filter with both a DSL modem (R45) jack and a standard phone jack. Ideally this filter will be plugged into a phone jack close to where the phone line enters the house.

Opinions vary, but most better installers will insist that every phone in the house should be plugged into its jack through a separate filter. If the wiring is very "clean" you can sometimes omit the individual phone filters and rely on the "master filter" to keep the DSL and phone signals isolated from each other; but an unfiltered phone that "wasn't used much a year ago" but is now more frequently used could explain the change in DSL performance - according to some of my installers.

AT&T has added different "tiers of service" for DSL since I first signed up. In my area the lower tiers are theoretically limited to lesser bandwidth than the ones that cost a little more. The level of service you subscribe to may have acquired sufficient traffic in your area to be more frequently "limited." While the service limits are sometimes expressed as "maximum (mega)bytes per month" the method of control often is lowering the "bytes per second." A neighbor downloading lots of movies (or porn) could cause the ISP to cut back the line capacity, affecting you along with the offender (in some areas where there's more traffic than on my lines?).

Cat 5 wiring is probably better for any parts of your wiring that carry the DSL signal. It shouldn't be required, but might be worth considering especially if you have fairly long wire runs. Cat 5 is constructed with twisted pairs just like the common phone wire, but each pair is (theoretically) separately shielded in currently available sub classes, along with another shield around the whole wire bundle. This helps to give more consistent "propagation delay" and less likelihood of crosstalk between lines (as long as you don't pair unpaired lines).

As described in the Wikipedia article on "Cat 5" (sorry I didn't save the URL last time I looked), prior to Cat 5e there was no real requirement that wire/cables advertised as Cat 5 actually perform as intended. Lots of "Cat 5" wire was advertised as "tested to ..." without any indication of whether the wire passed the test - and a lot of it apparently didn't pass. Cat 5e theoretically is required to actually be tested and to pass the tests; but it hasn't really been around long enough for many reports about how well the new requirements are enforced. (Buy only from reputable sources.)

If you have a router in your setup, the router may require something other than the default connection, and occasionally the ISP may flip you back to the default so that you have to do it over. My LinkSys router requires a "non default" POP3 configuration that was rather complex to set up with my first DSL connection, and resets required a couple of times when AT&T made changes required that it be (partly) done over; but AT&T (and/or LinkSys) updated their site to provide a "quick setup" almost painlessly by the time we re-connected after our recent move. The only problem was that the sales people (most of AT&T's websites) and "tech support" you can get to through ordinary channels didn't know where the required page is. Only the installers could tell me how easy it is now. (When someone tells you something that works, get their phone number and keep it handy. You've found the very rare AT&T support person with skills you'll need.)

John