Speech impediment
Chris has an unusual manner of speaking, due to the combination of his strong regional dialect and what appear to be genuine impairments.
History
Counter-intuitively, Chris's speech impediment appears to have worsened over time. He spoke relatively clearly and normally in his earlier videos (outside saying "listen" as "risten"), while his stuttering and other difficulties have increased significantly since then.
Likely causes include troll-induced stress and a lack of conversational company not already accustomed to Chris's speaking tendencies.
Chris's problems in verbal communication potentially stem from either the extended period of "muteness" he went through during his childhood (supposedly as a consequence of his treatment by the abusive babysitter) or his autism, a disorder which often results in speech difficulties. Chris's written communication generally tends to be significantly more coherent and grammatically correct than how he speaks. While writing, he has more time and freedom to determine what he says.
Nervousness in general may also be a factor with Chris's stuttering issues. His early videos in which he stuttered less and spoke more fluently, comparatively speaking, were possibly rehearsed and may have required several takes before the final version. He was also within the comforts of his own room and house, and possibly felt more relaxed than he would in a normal social situation. When conversing with outside parties, such as trolls or random strangers out and about, he may feel more pressured to be sociable and thus, trip and stumble over his sentences. Another possible reason for his degenerative speech habits might be the increasing levels of paranoia Chris has that every person he comes into contact with might be a troll trying to instigate him, causing him to be become less comfortable around people more-so than he already was.
Non-Symptoms
Slang such as "Y'all," and "'em," and contracting the "g" of words ending in "-ing" is typical in dialects of the southeastern United States, Chris's native region.
Chris tends to mispronounce names (such as Shigeru Miyamoto and Reggie Fils-Aimé) due to his ignorance of how they are actually pronounced, rather than impairments in his speech.
Symptoms
Chris's diction is naturally staggered, interspersed with pauses, deep breaths, "um," "uh," and "hm." The pauses and deep breaths, used in speech therapy, suggest that Chris has had his speech impediment professionally treated at some point. Chris's protracted style of speaking seems to make it easier for him to lose his thought, stray off topic, or forget to include certain segments of his thoughts. However, in the Flipnotes he has made, his speech is surprisingly fluent, with no "ums" or "uhs" This fluency is probably because he is being timed to do it, so there's little time to stop.
Chris also has a strong tendency to stutter. Although not the type of "machine-gun" stutter largely associated with the problem, Chris constantly has to repeat whole phrases, and he stumbles over certain phrases as he finds the correct words to best convey his ideas. This also could be because Chris finds the need to correct himself, and finds repetition to be the optimal way to deal with this. Ofte- usually this resu- this resu- thi- th- this ends up in Chris having to rei- repe- this ends up in Chris having to shuffle through many synonyms just to express a brief statement.
During the conversation with Kacey's dad, Chris claimed to pause before speaking in order to "spellcheck" what he is saying. This implies that Chris needs to consciously plan what he is going to say or else he is unable to say it; this suggests that Chris's difficulty with speech stems from poor contact between the speech centers of his brain and his cerebellum, a common characteristic of autism. Ironically, Chris clearly doesn't spellcheck his written work.
Like small children, Chris has a tendency to speak too quickly, run words into one another, and slur words. For example, Chris's pronunciation of "Ensykapeia Dramakka paysh".
Chris sometimes mixes up the pieces of multiple word phrases, resulting in statements like "harvey-dirdban" "rell-wested." [1]
Chris has difficulty pronouncing the letter "X" (/ks/ or /gz/), as well as related consonantal digraphs (/sk/, /st/), pronouncing it either as an "/s/" sound (elision) or as a "/t͡s/" sound (fronting). In addition, he pronounces the voiced "th" sound (/ð/) as "d", even when it's not deliberate (as in "da" standing in for "the"). This indicates trouble with consonants, known as articulation disorders.
However, given the above, it is most likely he has tachyphemia. This explains most of his speech and grammar usage.