Why do some people stutter only when they are nervous? Background: I don't stutter at least not always. For example, if I be reading out a article to myself, It is sure that I will not stutter even that I am reading it out loud.
The times when I do stutter is say when I am doing a presentation in class! or just saying something to my mother.The time always came up without warning. I will not know If I am gonna be stuttering reading a certain word until I actually reach it. It really creates a problem for me, and I can't find the reason for it..

Question:


Why do people stutter when they are nervous?
Interesting question! 

Assuming you are talking about stuttering, as occurs in about 1% of the population (Sommer et al., 2003), as opposed to non-pathological stammering during stressful situations that many more people experience, my answer is as follows:

Stress and speech disruption in stutterers are undoubtedly related (e.g., Craig, 1999). In fact, they may be causally related, as stress can provoke or aggravate stuttering, which in turn may cause more stress to the stutterer due to feelings of shame or loss of control. This in turn may induce more stuttering and may thus lead to a downward spiral, or vicious circle (Schwartz, The Stuttering Child) that may grossly disrupt speech. 

The underlying physiological basis of why stress provokes stuttering has been addressed by Weber &amp; Smith, (1990). It shows that speaking increases autonomic activity (elevated blood flow, increased heart rate, sweating) and that this sympathetic arousal was correlated with increased stuttering behaviors in stutterers. Interestingly, the sympathetic arousal of stutterers was found not to be outside the range of controls. The authors bring up several hypotheses as to why sympathetic activity may deteriorate speech fluency in stutterers: 


Increased autonomic activity has been associated with irregular breathing, which may affect speaking;       
Release of the stress hormone adrenalin may induce muscle tremor and spasms that can affect speech production;
Sympathetic regulation of afferent control of the sensitivity of sensory receptors (muscle spindles and mechanoreceptors) in the oralmotor, laryngeal and respiratory system may affect speech motor processes;
The gain of reflex arches (feedback) involved in the musculature engaged in speech may be modulated such that speech fluency is negatively impacted. 


These processes may aggravate the underlying pathology of stutterers (Büchel &amp; Sommer) and hence lead to more pronounced stuttering under stress.  

PS: Being a stutterer myself since early childhood, I support the behavioral consequences of stress mentioned in above studies by anecdotal experience :)

References
- Buchel &amp; Sommer, PLoS Biol 2004; 2(2): e46
- Craig, J Speech Hear Disorders 1990; 55: 290-4
- Schwartz, The Stuttering Child, Coping With Stuttering
- Sommer et al., Movement Disorders 2003; 18(7): 826*30
- Weber &amp; Smith, J Speech Hear Res 1990;33(4): 690-706