new DOCSIS 3.0 cable modem, wireless is now slow as molasses. router problem?

I recently upgraded to a DOCSIS 3.0 cable modem, Cisco model DPQ3212.  The end result is that I have very slow wireless internet now and can hardly get on Netflix anymore.  My router is a Cisco Linksys WRT160N (v1).  The router is old but it worked fine with my old cable modem.  I called Linksys support and they would not even troubleshoot me and instead suggested I buy their new $250 router.  Is there maybe a setting on my router that I need to change or is it truly "too old" to work with my new cable modem?  I feel like trying to get my old one back because there was nothing "wrong" with my internet before!  Please help. 

I did connect the cable modem directly to my computer and the speed test was about 35 mbps download, 15 upload (we are on the preferred tier) so that's why I think it must be a communication problem with my wireless router.