ITunes locks PC during large downloads

Question

This started happening about a month ago and there have been no major changes to my system (I've had this system for over a year).
PC running Windows Home 7 Premium 64 bit with SP1 and all available Microsoft updates installed.  iTunes is version 10.6.1
The sytem locks during downloads - usually they need to be large such a video podcasts and now a movie.  Downloads start and proceed at high speed to a point and then progress on the download stops and the indication of remaining time starts to go up.  For example, I was trying to download a 383MB video podcast and it flew along up to 62.1MB and showed 2 minutes remaining to complete in the status.  Then it went to 3, 4, etc.  After 2 minutes elapsed time, it was 7 minutes and after 20 minutes it was 2 hours. However, the indication of estimated completion time is the only thing on the system that is active.  The clock in the system tray stops almost immediately after the download progress stops nothing else is accessible - even Ctrl Alt Del and Ctrl Shift Esc do nothing,  The only thing I can do is physically power off and them it takes over 20 minutes to get back into iTunes as I have a large library (over 24000 songs and about 30 movies).
This has happen consistently in several locations with high-speed internet and I've tried turning off wifi and going hard wire.  Yesterday, I got a movie and the download to Flixster went fine and the iTunes download locked up.  I have removed and reinstalled iTunes and deleted the content in the download directory.  Also have unsubscribed and resubscribed to podcasts. Ideas on next steps would be appreciated.

Answer 1

Additional info - after 6 iterations of donwload/lock/reboot, I did successfully get the movie I bought (2.25GB of content) and it does play.
There are Bonjour errors in my application log:
System
Provider
[ Name]
Bonjour Service
EventID
100
[ Qualifiers]
0
Level
2
Task
0
Keywords
0x80000000000000
TimeCreated
[ SystemTime]
2012-06-12T11:41:38.000000000Z
EventRecordID
53865
Channel
Application
Computer
BLACKPEARL
Security
EventData
Client application bug: DNSServiceResolve(5c:59:48:4b:5b:29@fe80::5e59:48ff:fe4b:5b29._apple-mobdev._tc p.local.) active for over two minutes. This places considerable burden on the network.