I think you're making this too difficult for yourself - what's wrong with modulo arithmetic?

*(rainDuringHour210 + hourToday( *TZ300 - 1)) = mmB;

Any particular reason for this?

Why not the shorter, and more intuitive

rainDuringHour210 [hourToday( *TZ300 - 1)] = mmB;

?

You need to look at the scope and your handling of "buffer" - it is kind-of chaotic, particularly this bit:

Code:

    for (int x=0; x<bufferCount ; x++) {
      buffer[x]='\0';
      }

Hint: at this point, buffer is only one character big.

THE CODE:

At this point, you're supposed to hit the "#" button on the editor''s toolbar, between the printer icon and the "insert quote" icon-thingy.

Nothing calls "readToneData"

// PROBLEM APPEARS [glow]TO HAPPEN[/glow] HERE            

Yet you're still not posting the code. :o

Any explanation for this mystery?

Tell us what "sampleCount" is, post your code.

You don't say how you've got your buttons wired or what sort of buttons they are, but I notice you're not using the built-in pullups.
This may cause you problems.

Use "print", instead of "write"

It definitely wasn't as slow as 16[glow]m[/glow]Hz though !  

I only posted part of the code

Well, thanks for wasting my time.

Code:

while (Serial.available() >0)
{
 for(int i=0; i < 4; i++) {

This isn't going to work.
1 is > 0, so what happens after one iteration of this loop?

Can anyone see anything terribly wrong with my code?

You mean apart from that fact that you haven't posted all of it?

What does "setup" do?
Where is the code for "read_adc"?

Disable the Arduino's autoreset feature.
http://www.arduino.cc/playground/Main/DisablingAutoResetOnSerialConnection

You wait 32 microseconds for all the data to arrive

Not even close

No mystery, but not recommended.
It causes a delay of 32 milliseconds (and therefore limits your update rate to about 30Hz).
In those 32 milliseconds, you could have transmitted over 350 characters at 115200 bits per second