Loss of internet access but not LAN

I'm a network tech going on 13 years now and have an issue that is completely stumping me so I thought maybe you guys/gals might be able to provide me with some new ideas. I'm having a problem with a network in a 100+ year old building with some users randomly losing internet access but they still have LAN access. Here is the network setup:

• Dell PowerEdge 2900 Windows 2003 Server
• Cisco firewall PIX 501 series
• D-Link 24-port unmanaged switch (no patch panel, all cable runs go directly to switch)
• 11 Windows workstations (XP Pro 32-bit and 7 Pro 64-bit)
• 3 wifi access points
• Camera system DVR
• MFP Printer
• Postage machine

DHCP is on the server, nothing else hands out IP addresses. A couple of months ago a user reported they were unable to access the internet. I investigated and found indeed they could not access internet but had a local IP and could access everything on the LAN. Here are the steps I've taken so far:

• Cleaned PC of all viruses - no change
• Toned out cable run from the user's jack to the switch - tested OK
• Replaced NIC card in PC - worked for a few days then back to same result
• Connected known working laptop to user's jack - same result, LAN access but no internet access
• Physically took user's PC to switch and connected it directly - LAN and internet access, so now I'm thinking the cable run must be pinched slightly somewhere
• Replaced entire cable run from user's jack to switch - no change
• Again connected known working laptop to user's jack - no change
• Bypassed firewall by plugging switch directly into modem - no change
• Connected server's second NIC to the switch - no change

Now another user's internet access only is going out temporarily at random times, and now I've just received a call that the MFP printer cannot be printed to. It seems like this issue is growing and I've run out of ideas. Please help!