Deleting all songs in a given iTunes playlist using Applescript
I've had the exact same problem. There's more and more AppleScript bugs in each version of OS X.

If you just want to delete all tracks on a playlist, single expressions like this don't seem to skip any tracks and they're also faster:

tell application "iTunes"
    delete tracks of playlist "untitled playlist"
end tell


If you only need to delete some tracks, something like this might work:

tell application "iTunes"
    repeat
        set tr to tracks of playlist "untitled playlist"
        if tr is {} then exit repeat
        repeat with t in tr
            delete t
        end repeat
    end repeat
end tell