Anytime you get the dreaded "No PPP login service found - unable to connect to login service" it is typically a problem with the ISP network. I say "typically" because an incorrect keycode in the modem can also cause this error. However, since your service works other than at these times you probably need to contact your ISP and report this problem to them.
The error basically means that although your modem is able to hear the dsl signal in the line (meaning your connection is good all the way to the RT or CO) it is not able to communicate with the radius server for broadband authentication (either it's getting lost in the ATM switches or the ISP network router is not configured correctly/stably).
Be sure to mention that this is NOT the first time this has happened. They will be able to run some tests to determine if this is a software issue (typically correctable over the phone) or a hardware issue (which would require some maintenance to their equipment). The fix is usually quick.