Wireless Clients lose connection intermittently

Weird issue – have had Cisco 1240 AP’s (3 of them) and no issues for several months. All of the sudden, I have a roomful of users who lose their connection to the AP. The signal strenth is fine. We can ping the AP, then we can’t – and i am about 20 feet from it. We replaced the AP and the antenna, changed the network jack and the patch cable. Still have intermittent issues. Checked the PIX which hands out DHCP which we suspected might be the issue. Nothing weird going on there. Called Cisco and they said upgrade the PIX IOS. But the IOS was fine for the previous 3 months. Ideas or suggestions???