Select query ignores index on large table

Hello, I'm experiencing a strange behavior running a simple select query on a table that has about 12 million rows. Specifically, changing the "LIMIT" value seems to change the execution plan but the result in one of the cases is unjustifiably slow, as if it ignores all indexes.

The table structure, the queries used and the results are here: http://pastebin.com/fn36BuKs

Is there anything I can do to improve the speed of this query?

Are there any configuration settings that I should change?

Thanks,
Mike