Problems with SonicWall TZ170 - Verizon DSL Router

I am helping a friend setup their network at a small non profit.  They have a Sonicwall TZ170 firewall and have new DSL service from Verizon.  Verizon provides a Westell DSLModem/Router with the connection.   Initially  we were running with double NAT.  The Westell router LAN config has DCHP turned on.  The LAN port of the Westell is connected to the WAN port of the Sonicwall.  The Sonicwall is configured to use NAT with DHCP client for the WAN and to act as a DHCP server on the LAN.  This seemingly worked fine.  We could get onto the internet with no problem in most cases.  The trouble we've seen is that we can't get to certain websites and have trouble logging on to websites that require authentication (some not all sites).   We can ping these sites fine but a web browser just hangs when trying to access the page or complete the login authentication.  It's happened on both Firefox and IE.  As soon as we remove the Sonicwall from the connection and have the desktop computer go straight to the interent through the Westell dsl router, those same website work completely fine.  I tried to adjust the Sonicwall to use Transparent mode, where the sonicwall DHCP server was turned off and the Westell acted as the DHCP server.  In that case we could still browse the internet but we had the same weird problem with certain websites.  In effect, transparent mode did not solve the problem.  So, it seems there is some issue with Double NAT (or maybe not since transparent mode didn't work either).   For now I've just removed the sonicwall from the connection entirely (the Westell has a firewall built in as well) but for obvious reasons we'd like to use that.   Any suggestions would be very helpful.