Common problems with USB converters I'm afraid. Many of them don't understand the extra key between left shift and Z. And almost all of them reset from time to time, dropping characters. Quite unusual for one to completey die periodically though.
And with the Dell, it's harder to recommend a converter, because the blue cube doesn't like them. IIRC, the 'good' Belkin does work correctly with them...
I have problems with both the belkin ones I have. They will "drop out" for a second or two, then come back. It's not a big deal, but can cost you if gaming.
There's one good version of the Belkin, and there's two or more bad versions
Sorry I can't recall the version numbers, but a search should find it. Shame the wiki's gone, it was there.
edit: F5U119vE1 is one of the bad ones. All the different version begin with F5U119, so that will help when searching. Google tells me there's an F5U119-E and a F5U119-NW as well - possibly it's the -E that's the good one, but I can't remember for sure.