(To be fair: I am and always have been someone who works best and puts out the most effort when challenged. If I am not challenged I put out mediocre effort. I know this about me.

You just repeated the exact comments that have given to me by several people. I don't get out of first gear unless the problem presented has a level of difficulty to peak my interest.

I have made that perceived weakness into a strength by seeking out difficult problems and solving them, if nothing presented itself I went over and above to come up with problems others are having and 'fixing' those. Now this may be specific to our particular jobs, working as a developer I may have more scope to do that but it should still be something you could try (If you want to of course).

The other problem I had which is the hardest for me to overcome is self-promotion, letting others know when you have done a good job and preventing others from stealing your thunder (bosses, fellow co-workers included). For me that was the most difficult thing to change but its a life skill that yields the most return.