Is it happening all the time or only sometimes?
You may get problems like this when the videos are not cached (such as new videos, or rarely viewed videos). Like many internet services, youtube uses distributes servers to cache their most popular content, and not all content is cached.
You may also get this when youtube is doing maintenance (which they usually announce, but not always), since this may mean that some of the servers are offline.
Do the videos stop paying even though they are loading (based on the progression of the red bar) or do they also stop loading?
The latest version of flash for IE is 10.0.22.87
Is that the version you have?
I do not use IE7 and have never installed it because it was incompatible with the SAP client I use for work, so I cannot help you on that end.
However, as a general rule with problems of this type, you may also have problems with you own computers cache/cookies etc, and you may want to clear your local cache, cookies etc.
Several places on the web also state that if you have Google Web Accelerator that this could cause the problem, and that you should get rid of it.
Here's some advice from yahoo answers and other sites on similar questions:
http://answers.yahoo.com/question/index ... 741AAAxU1t" target="_blank" target="_blank" target="_blank
http://duggme.com/2009/05/why-when-watc ... -stopping/" target="_blank" target="_blank
BrianIs
AtYou