Here are two transient factors I know (but there definitely are more!):
- The color red (either presented as a color, or as a word) induces an avoidance motivational state, leading to an impaired IQ test performance 
- Stereotype threat: Inducing a negative stereotype of a group (e.g.: Women are bad at math) causes members of this group actually to perform worse. This works with gender, race, age, etc.
: Elliot, A. J., Maier, M. A., Moller, A., Friedman, R., & Meinhardt, J. (2007). Color and psychological functioning: The effect of red on performance in achievement contexts. Journal of Experimental Psychology: General, 136, 154-168.
: Maier, M. A., Elliot, A. J., & Lichtenfeld, S. (2008). Nonconscious avoidance motivation mediates the negative effect of red on intellectual performance. Personality and Social Psychology Bulletin, 34, 1530-1540.
: Gerstenberg, F., & Imhoff, R. (in press). 'Women are Bad at Math, but I‘m Not, am I?’Fragile Mathematical Self‐concept Predicts Vulnerability to a Stereotype Threat Effect on Mathematical Performance. European Journal of Personality.
: Smith, J. L. (2004). Understanding the process of stereotype threat: A review of mediational variables and new performance goal directions. Educational Psychology Review, 16, 177-206. doi:10.1023/B:EDPR.0000034020.20317.89