limit order by performance issue

Hello,
   I'm trying to do a simple SQL query over Postgresl 9.0 running on Ubuntu.

I have a large table (over 100 million records) with three fields,
id_signal (bigint), time_stamp (timestamp) and var_value (float).

My query looks like this:

select var_value from ism_floatvalues where id_signal = 29660 order by
time_stamp desc limit 1;

So I want to select the last value from a determinated ID (is_signal).

This query runs FOREVER, while if I delete "limit 1" it runs instantly....

Any help?

Regards.