[tor-relays] running relay severely degrades network performance eventually

Hello list, every time I run a relay on a Comcast cable connection, after a while (a few hours to days) other network operations such as DNS lookups slow down to such an extent that the network becomes unusable. Unfortunately I don't know how to exactly pinpoint what the problem is, but found that the only reliable solution at such a point seems to be to reset the cable modem. Is that a known issue? Could you suggest how I can diagnose this more accurately? Thanks, nick