How large is your TNSNames.ora and how fast do you type? Mine has over 100 entries, and I modified it temporarily to include ranges of entries like:
I was able to jump around randomly typing up to the first 3 letters without too much of a problem if I typed quickly. Sometimes it would seem to stick a bit (switching to a different first letter, then switching back helped), and keep in mind that repeating the same second or third character (rapidly) just scrolls through that range.
Hope this helps. If there is something specific to your environment you suspect may be a factor, please let us know.
SQL Developer Team