Ok we have eliminated the phone line. That is good, because it is outside your control

Next:

1. Check the cable from the modem to the phone socket. Preferably borrow another one and try that. I have known intermittent connection problems and cut-offs to be caused by a damaged/faulty cable. Also check that it "locks" into the modem and you cannot pull it straight back out. Same at the wall socket end. (= faulty cable)

2. Try to borrow an external modem and test with that. (= faulty modem)

3. I am assuming that your modem is internal? If it is external you could take it to someone using a different ISP and see if the problem is still there (= faulty modem)

4. Do a google search for "modem testing software" and try that. Preferably set the "external loopback test" to a number other than your ISP (=faulty modem)

If all these are OK my conclusion is that you need to change ISP? Is it possible to get a "free trial" with another provider, and compare results?

Good Luck