Frequent internet disconnects

For about a week, I've been dealing with frequent disconnects on my internet.  I'm using a Motorola Surfboard modem and an Asus router.  I replaced my router with an old Linksys and still experienced problems.  I've also tried moving the entire setup to the cable line outside my house, to confirm that it wasn't bad wiring in my walls. The problem is solved each time when I restart my router, but I've had to do that several times an hour in some cases.

I've found my model signal strength, as suggested in the Connection Troubleshooting Tips post.  Almost everything appears within the acceptable range, except one of the four channel bands has a downstream signal to noise ratio of less than 30dB.  One of the others does occasionaly dip below 35dB.  

Any suggestions on what might be wrong with my service?

Thanks!