Workstation loses internet

Have a couple of workstations that have a DHCP address and intermittently lose (only) internet connectivity. Their LAN connection remains stable. They can ping the gateway and the firewall, but are unable to ping an external IP address past the firewall. At first I thought it was DNS but they can't ping an external IP address. The outage lasts 2-8 minutes and then it's fine. I've tried searching for duplicate IP addresses on the LAN but don't see any. If I assign them a static IP address, outside the scope of DHCP it seems to resolve the issue. I can also ping their workstation from any other workstation on the network, however, NBTSTAT won't return a host for those affected users.