Just out of curiosity: Do you know more precisely why you lost against lower rated opponents? Did you lose in the opening, middlegame or endgame? Did you lose because of time management issues? Also, what do you mean by long time to warm up? What changes for you from "not warmed up" to "warmed up"? Is it level of focus, some mental block dissapearing, or something else?
If you answer questions such as this you may find some things you can improve upon, which may be more important than anything you can read in a book. It may not even be directly related to chess, but it could still affect your chess in an unexpected way.
If you're already an FM, I have a feeling that you already have a good idea of areas of your games that are weak, so I would suggest reflecting about that and then add some details about your specific chess strengths and weaknesses. You may, for example, be an opening expert who isn't really as consistent in endgames as is expected of an FM. Then you may want to study some endgame litterature. Or it may be the other way around; nobody here can know that better than you.
A general advice though, is to try to contact an experienced chess coach, and also, if you have the possibility, talk to strong players(FM+) about ideas for improvements. One other thing you should probably do regardless of your specific situation is to write down and analyze your thoughts during a game as soon as possible after a game. Then it might get easier to pinpoint precise answers to questions like the ones I asked in the beginning, which may hold the key for your improvement plan.
Also, if you're physically unfit, but able to exercise, I suggest you build up some basic fitness. This would most definitely affect your chess in a positive way.
Good luck with your IM ambition. I hope you find a way to get there.