It was controversial but eventually the DSM critieria for borderline personality disorder was altered to include the criterion of 'brief psychotic episodes during times of stress'. it sounds to me that your experiences would fall under that symptom. i get this stuff too. mostly when i've had a lack of sleep and / or when i'm particularly stressed out. while it is considered a 'brief psychotic' kind of a thing it is supposed to be different from psychosis in that it is shorter lived and reality testing tends to remain intact (or if it doesn't the episodes tend not to last more than a couple hours to a couple days).

the flashback experiences of PTSD can also resemble hallucinations etc.

i don't think it is synasthesia - that is a very technical notion, typically. i guess there is a general understanding of it which includes hallucinatory experiences but a clinical diagnosis of it is a very different matter. Varieties of stroop tasks are used for diagnosis, see, for example:

http://en.wikipedia.org/wiki/Synesth...e_verification