Has anyone any experience of any problems with the Hitachi AA analyser (8800) like blockages in the plumbing, distorted peaks, elution time shifts etc etc?

what is the best sample preparation for plasma and urine?
what is the best internal standard?

I have never used a Hitachi AA analyser. However, I have used a Beckman AA analyser and the internal standard we used was alpha-aminobutyric acid (AABA).
When I used the Beckman AA, any blockages were usually attributed to the following: after the injection had complete, the column needed to be washed with water to remove the post-column derivatizing agent (I want to say phenolthalene, but I'm not positive). The column was then stripped with NaOH. If all the derivatizing agent was not washed off prior to addition of the NaOH, the NaOH would cause precipitation in the column and the lines.
I also remember that the instrument was very high maintenance and if proper care was not taken, there could be numerous problems, including the ones you have listed above.
Sorry I could not be of more help.