Hi,
If I recall going way back a few years ago, I tried something very similar with a Allied Telesis TFTP server software on my PC trying to upgrade IOS on a remote router.
Wireshark indicated that my PC was actually at fault and not streaming the UDP traffic to the router. The UDP stream just stopped, go figure :-)
My resolution was to try another freeware TFTP server, in my case it was the freeware 3COM TFTP server, and all worked ok. Wireshark capture confirmed that the new server streamed fine to the remote router.
So what i got out of that experience was;
1. I learnt how to use wireshark.
2. replaced my old faithful TFTP server software and try another TFTP package and try the upgrade again.
regards Dave