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I think you do. Unless you actively do things to stop it from happening. Some people including myself seem to hit a plateux and level out OTB. I think the best way to overcome a level trend in your rating is to shake things up now and again. Learn a few different Openings each year, dont play the same ones over and over. Read differnt books, basically dont let yourself get in a rut. Also a break from chess if only for a few weeks once a year means you come back with vigour, and with a few of those cobwebs shaken off :-)
Even though I am a novie player, I have experienced what he's talking about. It took learning new openings and really really working on my openings (and everything else: reading new books, training aids, etc) to start working towards the next level. It can be done (Even though I'm still a lousy player. However, I love the game and keep plugging away at learning it).
Do you stop learning and getting better after reaching a certain level in chess?