I don't think your problem is specifically choking, and I don't think you should be overly concerned about it. Part of what makes a player stronger than you isn't just their chess knowledge, but consistency. Strong players are able to play entire games without mistakes (tactical blunders here, everyone makes mistakes to some degree), but as you move down the rating ladder that decreases.
A few weeks ago I played a 1300 player that played very well. For about 30 moves, neither of us made huge mistakes and the game was pretty equal. I was getting nervous, really starting to sweat. Then in two moves he completely collapsed, dropping material on one move and allowing a forced mate the next. You could say he choked the game away, but more accurately I think he's just not to the point where he can play every move at a high level.
Like in golf, great players don't hit amazing shots every time. But they consistently hit good shots.
Don't over analyze what's going, just keep playing and studying and you'll see your results improve.