Lost Internet Connection when Cable Modem Connected to Router

I've got a Mediacom cable modem (RCA by Thomson model DCM425).  My internet connection is pretty reliable when I've got the modem connected directly to my desktop PC.  When I connect the modem to my router (Linksys ES4200), and my desktop computer to the router, the internet connection becomes very unreliable.  I often lose connection within 30 minutes of connecting to the modem to the router.  At that point, I have to unplug the modem and restart the desktop PC to get a connection.

Wireless is enabled on the router and I think when I access the router via wireless on my laptop may be part of the problem.  It seems that I'm much more likely to lose the connection when I'm using the laptop.

I've called Mediacom (my ISP) and they say everything checks out on their end (although they're sending a technician out).

I could try disabling the wireless on the router but that defeats most of the purpose of the wireless router.  

The router is less than a few months old and everything has worked flawlessly up to about a week ago when the problem began.

Suggestions?

Thanks,
darin