I should have said they are all wireless.
can you hardwire one and see if the problem goes away? It might just be a problem with the DSL service proviced.. I have wireless and every once in a while I have problems!
If you can bypass the wireless (direct connect instead) and see if that makes a difference. If it does, then perhaps try configuring your wireless to use a different channel (perhaps some neighbor turned theirs on using the same channel as you).
I had a problem a while back where I would lose my internet connection whenever I got a phone call on my land line.
Turned out I had a cordless phone on 2.6ghz, and I was also using 2.6ghz WiFi. Upgraded the access point to 5 ghz, and the problem was solved.
Bear in mind WiFi can also suffer from bandwidth overcrowding, depending on the number of neighbors you have.
As others have suggested, I would plug the computer into the modem directly, bypassing both the questionable router and the questionable WiFi, and see if things improve. Next, I would plug the router into the modem and the computer hardwired into the router. If things are then OK, it would point to WiFi problems.
If WiFi's the problem, try to get the WiFi onto a different channel or try a different access point. In my case, I added an Apple AirPort, operating on 5ghz, in bridge mode below the D-Link router plugged into my Verizon FiOS box (and providing WiFi on 2.4ghz). Also, I eventually replaced the 2.6ghz cordless phone with a $20 5ghz model. Currently, both WiFi networks are OK (the 2.4ghz provided by the D-Link and the 5ghz from the AirPort).