|Is the modem a combination unit? Which is to say, it's a modem and a router?|
If so, upgrade the firmware if it's not up-to-date.
If it is, try resetting it to factory defaults.
If neither of the above works, then call your ISP, you need it replaced.
If it's just a modem, then call your ISP's support line and have them check the modem out. If they say they can't find any fault with it, insiste they send you a replacement regardless as your unit has to be faulty, whether they can see it or not.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.