But unless we change out the modem we can't be sure it's not the modem. I can see why some might think "it can't be that" but how else to test this other than a swap? We also have learned of failing DSL filters that can be temperature sensitive and behaving on a timed schedule. There can be dew in the hours you note and causing a failed line.

Your ISP is not the owner of the lines so they won't send a crew out to check.

Keep an open mind and do what they ask to get past their item list and hurry along to the real fix. Telling them "no that's not it" only slows down their process.

Bob