I really think you need to stop and think what is going on here.
In your last post, "readSerialString" returns as soon as there are no characters in the rx buffer.
Imagine I type a 'P' and then pause for 5 or so seconds.
The P is read out of the buffer, put into CString and the string terminated correctly. Now wait 5ms, and have a look at the rx buffer.
The rx buffer is empty, so exit "readSerialString", and call "processSerialString".
"iBufferIndex" is == 1, and the first element of CString is a P.
Overwrite the P with a space and call "atoi" on a correctly terminated string, which contains only a space character.
See where this is going?
All the "delay" is doing is allowing you time to type another character and transmit it. (at 9600 baud, and 8n1, a character takes 1.04ms to transmit).
Bottom line: you HAVE to have some sort of terminator character for this to work reliably and correctly under all conditions.
You have to test for that character and only when you have it, process the string.
[edit]Or have fixed-length commands[/edit]