I had a similar problem. I added a short sleep and an additional ipconfig /renew to init.bat and the problem was solved.
Maybe not in main context of your probloem, but I'd like to ask you about that IBM machine of yours. Have you had any problems booting WinPE2.0 ob IBM bare-metal? I'm trying to provision Win2003 to IBM x366 and with WinPE1.6 it's OK (I mean, regarding booting - later on we've run into the IP issue just like yours, and solved it with ipconfig /renew in win2k3.bat), but when I try to boot WinPE2.0 the bare metal only downloads bootmgr.exe from tftp and hangs - blank screen, no activity regarding bootimage downloading, nothing. As far as I can tell it may be related to a rather old bios (1.09 vs. 1.12 latest) but machine is clients' not mine in a remote datacenter so bios upgrade is not an option.
If you enable debug in your tftpsvr.cf you can see what exactly the bare metal server is pulling down, what requests and what it receives.
Change the line:
log4j.rootLogger=INFO, R, C
log4j.rootLogger=DEBUG, R, C
Check out the tftpsvr.log while you are booting the bare metal box. All files are located in the PXE/br directory. Hopefully that can help you isolate down whether or not the boot image is able to be transferred to the bare metal box or if its the winpe2.0 boot image itself that is having problems starting.