Resolution Problem

I have a very inconsistent resolution, i have hooked my laptop up to my 40" sony bravia. The max resolution supprorted by the TV is 19201280 but my laptop usually only puts out about 1360768. I have managed to get 1440 by something but and last night i even got 1600 by 1200. I don’t understand how the slider under display settings determines the max out put, at times it is 1360 by 768 and yesterday it was over 2000, ofcourse my tv cant support that. 1600 by 1200 was nice, but as soon as the computer restarted, i lost the resolution. Can anyone tell me how to get a consistently good output?