Been using the Zoom's for a couple of years now, had an H4 on loan, got an H2, kept it a couple of years, sold it and got an H4n recently. I've never seen this problem. Admittedly, when I know I'm going to be recording for several hours or more non-stop, I'll use a high bitrate MP3 format, though.

Maybe that's the fix...

I do reformat my card before any big session, though. That might be part of it.