Hmmm.... doesn't make much sense why you'd see two different responses unless there was a PXEClass there based on the MAC.
Given how improbable this is I would say your TFTP server is missing a file that was once there. If you're using RIS this file would be OSchooser\i386\startrom.com. If you're using pxelinux obviously this would be pxelinux.0.
I would verify this directory structure and file(s) are present in your TFTP server's root/boot directory.
Other than that double-check your DHCP config for the proper boot server hostname/IP address (option 66) and check the boot filename (option 67).
Another possibility is that a second TFTP server is running on the same server (assuming your boot server name is as it should be) and using a different TFTP server. Shutdown your main TFTP server and do a netstat for port 69 UDP open to see if you still TFTP open even after it is supposedly shut down.
If the DHCP boot server specification and boot filenames/directory structure is there as it should be the only other possibility I can think of is you're getting IPs from a subnet you do not expect and thus possibly getting unexpected PXE responses. Of course you can tell what IP you're getting when you boot via PXE.
If all this fails and if you really want to tell what's going on setup Wireshark on your RIS/TFTP server and watch the TFTP traffic when PXE booting. You'll get a very sure and clear idea of what boot file the PXE client is requesting. More importantly if you see no TFTP traffic you definitely know then that something is wrong with your DHCP setup.
And if you're dead sure you do not have spanning-tree/etherchannel enabled on your ports the first error might point to an exhaustion of the DHCP IP pool space though if see the error above (PXE-T01 File Not Found) definitely points to DHCP working - unless the working piece is intermittent.
