many 404 crawl errors in blog

In Google Webmaster Tools, I have over 200 crawl errors (404 not found). The great majority have URLs that look like this:

http://theboomtownrap.perthpunk.com/wordpress/?p=665&cpage=1

When you click on these URLs a notice comes up from WordPress stating:
“This is somewhat embarassing, isn’t it? It seems we can’t find what you’re looking for. Perhaps searching, or one of the links below, can help.”

Also, when you click on any of these ‘bad’ URLs it brings up the following URL address: http://theboomtownrap.perthpunk.com/wordpress/comment-page-1/

I asked my host Tech Support for some assistance, which they were unable to give, stating that to investigate and remedy the issues involved would require an in-depth knowledge of WP, which they don’t have. However, they did provide some explanation of the above, as follows:

“The 404 error is related to the “cpage” argument.
When you access: http://theboomtownrap.perthpunk.com/wordpress/?p=665&cpage=1
you are actually looking for the first page of comments on post 665.
If you access: http://theboomtownrap.perthpunk.com/wordpress/?p=665&cpage=2
you would be looking for the second page of comments on post 665.
Since there are no such pages (there are only a few comments on the specified post) your blog returns 404 errors. The “cpage” argument is a standard one used by WordPress. Apparently, Google has indexed your pages with this argument. We do not know why Google did this, and how they found these specific links. “

Well, if they don’t know, I sure as hell don’t either! Can anyone throw any further light on this please? Specifically, are these types of 404 errors likely to be seriously affecting my blog indexing, and if so, how would I go about fixing them please?

I should add that my traffic suddenly dropped by over 70% immediately after I changed the default WP permalinks structure to a ‘pretty’ structure (about 2 months ago now), and has never recovered. I am wondering whether all these 404 errors could offer some clue as to why my traffic remains so drastically reduced.

Sorry about the long post, but I figured it was best to give all the info that could be relevant.

Cheers
Ross