One laptop connects 90% of the time to a RVS4400N and one only connects intermitently.

Question

This is a confusing scenario so I'll try to be as clear as possible.
I have a network with the following configuration:
A Linksys wireless router (WRVS4400N) is connected to a Verizon Westell DSL modem. (Downstream speed is 1.5Mb/s.) A client based app. connects to a server that houses data for the client on the Internet.  Two laptops are connected to the network via WiFi.  A Vaio can connect consistently to the Internet and run the app with little or no problems.  However, a Toshiba only can connect about 10% of the time.  When running the app on the Toshiba it eventually times out, or runs very slowly. (it's a connection problem not the client running slowly on the laptop.)
Here's the confusing part: the Toshiba can connect, using the client app, when on another wireless router, just not on the WRVS4400N router. 
To review: On the WRVS4400N router the Vaio can usually connect to client app's server.  Toshiba usually can't.  However, Toshiba can connect when using another wireless router. 
I can't figure out what's wrong.  If I connect the Toshiba using a CAT-5 cable directly to the DSL modem it works fine.  And when I connect it to a port on the WRVS4400N router, via a CAT-5 cable, it also works fine. 
I was thinking it had something to do with a keep alive timer on the Toshiba but it just appears that the connection the Toshiba has to the Linksys wireless router is slow.
Ideas??

Answer 1

Are the adapters 802.11g, 802.11n? Or other?  Which wireless adapter is working and which is not.  I'm assuming the Real-Tek is not.
How is the wireless setup on the WRVS4400N?  Which mode are you using, i.e. B/G/N-Mixed or other?
Within the WRVS4400N's web interface, if you go to the Wireless tab and select Advanced Wireless Settings, what settings are active in the WMM and IOT Mode drop-down menus?  You can dry to disable WMM or enable IOT mode if you are using 802.11n adapters and see if that helps.