The thing that makes my rating fluctuate (although never as much as +-200 points like that though) is if I'm rested and not distracted I play better (of course) but the tricky thing is when I'm tired or have other things on my mind I don't feel like I'm playing worse, but in the end I miss more stuff.
So try to be objective about why the winner won, and why the other guy lost. If a 1650 guy drops a rook or walks into a mate, you're not suddenly 1650 strength. If you blunder and lose against a 1300 you're not suddenly 1300 strength, even though in both cases your rating has changed.
When you're on your game and not making more errors than usual, what 100 points of player gives you a tough evenly fought game? That what I ask myself. And if I happen to luck out and beat some player rated 100-200 points higher than what I think my strength is, I don't consider myself that new rating even if my rating went to the next 100s because of it.
Of course I don't get excited or depressed due to my opponent's rating, I just reference it with his general level of play (even if they later drop a rook, I know about how strong their other moves were). So it may be different for you if your heart starts racing or something when you play the 1650 people, but 1450-1650 is a noticeable gap in playing strength, if you really think your strength of play fluctuates that much you should probably work on calming down and focusing on the game and forgetting the rating.
I have watched my blitz rating go from 1600 to 1350 and back up again. Right now I am in a losing streak: I can't even defeat people with scores higher than 1500. But just a month ago, when my rating was in the mid 1500's, the odds I defeated a 1600 were 50-50.
I have insufificient information at hand and I realize that must be more variables than I am taking into account, but it seems to me that within a given ratings range (Ie: 1450-1650), the likelihood that I will win a game depends more on my knowledge of my rating and the other person's rating (and this is probably also true for the other person) than our actual relative strength as chess players.
I could be wrong, but that might explain why when my rating is relatively high (1550+) I can easily defeat most 1500's, but when my rating is relatively low (1400s) I can't even defeat a single 1500. At times I wonder if the outcome of the game becomes a self-fulfilling prophecy once the person with the lower rating subconsciously convinces himself that he is going to lose and the person with the higher rating that he is going to win.
Of course, this seems to apply mainly in cases where the strength of the chess players is within the same range; when it comes to significant disparities in ratings the odds that the weaker player will lose are very high no matter what.