Try to ensure UPnP is enabled on your Linksys device; Linksys UPnP setting (click on link)
The UPnP if it is enabled on your device might be unreliable. In which case, a firmware upgrade for your model might be recommended.

Scratch! I just googled your model & found these comments about your model modem being reknown for locking up; see the user comments starting from about half way down of this page http://www.amazon.com/gp/product/B00...lance&n=172282 Particularly Alan Krueger's feedback.
It might be wise to seek a firmware update for your model!