Has this ever worked on this client? Are you in a location with other wireless interference? Have you tried VPN connectivity on a wired connection? Is there an update to the mobile wireless access software? Is there a firmware upgrade for the integrated card?
Note that a lot of VPN software is very sensitive to routing changes and dynamic IP address changes (this makes the “trusted” network connection become “untrusted”). So, if the wireless connection is intermittent/unreliable, then it is possible that the VPN connection is deauthenticating or losing routing information causing the connection to drop.