Thanks for the quick response!
The Wireless drivers are up to date (last version available at the Lenovo website) and the problem occurrence is random, the laptop is connected to the WLAN and simply disconnects and doesn't connect back. The point is, it only happens with the laptops using the Broadcom cards. We have several other laptops here that use Realtek or Atheros cards, for example, and the same thing doesn't happen, they work just fine and we never had an issue.
Regarding the firmware update, I'll consider doing it. But we have been having the same problem with the Broadcom cards in previous versions too.
Is there anything in the WLAN configurations that I can change that could be causing the issue?