Serial port eof errors

When I read a few megabytes over a serial link, the file closes
after about 6MB, sometimes. Might be related to PC load. Windows XP. 
Is this method OK?


    fconfigure $fi -mode 115200,n,8,1
    fconfigure $fi -buffering line -blocking 0 -translation binary
    fileevent $fi readable [list [info coroutine] read]
    while {1} {
        set line [yield $ret]
        while {1} {
            set err [catch {set bytes [gets $fi line]} msg]
            if {$err || [eof $fi]} {
                set done 1
                puts "*** Read failed, EOF"
            }
            if {$bytes < 0} break
        }
    }