Steve - over the last few days I have had a similar issue. I have two DSL lines from Frontier. MY primary line is fairly stable but my secondary line is not. I loose connectivity during the evening hours. There are times that I can pull PPP connection but will not pull an IP address. Then there are other times where I am not even able to pull a PPP connection. To set others minds at ease, I have three spare modems and they all exhibit the same problem and they all work without issue on my primary line. They are currently working the issue.
Put a call into Frontier, just give them the symptoms and don't add anything else to the report. Let them work the issue. You will, more than likely, have to become a squeaky wheel.