it sounds like a browser issue.....perhaps its timing-out on you! you're not on "dial-up" are you....lol (just kidding)
i was having a lot of issues when IE7 first came out, but i think they've since fixed any of those quirks and the drag/drop works fine for me now.
not to go to the "drastic" spectrum, but viruses wreaked (sp) some MAJOR havoc on my computer causing all sorts of issues! start small though, what browser are you using? and is it "up to date" with the most recent upgrades and such??
i found that when using the new version of IE7 (before wiping my computer out due to various viruses)...that my computer would "time out" and give those kinds of error messages if i tried doing "too much" at one time. i'd have like 8-9 tabbed windows opened on one browser...try switching between them during processes and found my computer would "freeze" or get stuck a lot...
regardless, i'm getting into more serious "issues" when perhaps all you need to do is download the latest version of your browser or edit your internet options or something.
hope i didn't confuse you...lol. good luck and let us know about the "browser" you're using.
cheers,
kathleen