onclick submit only submits once (more than one submit button)

[Trinity-Links]

I have created some online software which uses form data to submit to another page.

The problem is that I have other submit buttons in the same form which submit to different pages (depending on which button is pressed).

The first submit button works fine but all the others only work once.

<input class="cssbutton" type='submit' value='Button1' onclick="wbform.action='page1.php'; target='mainFrame'; return true;">
<input class="cssbutton" type='submit' value='Button2' onclick="wbform.action='page2.php'; target='_blank'; return true;">

<input class="cssbutton" type='submit' value='Button3' onclick="wbform.action='page3.php'; target='topFrame'; return true;">
<input class="cssbutton" type='submit' value='Button4' onclick="wbform.action='page4.php'; target='topFrame'; return true;">

<input class="cssbutton" type='submit' value='Button5' onclick="wbform.action='page5.php'; target='_blank'; return true;">
<input class="cssbutton" type='submit' value='Button6' onclick="wbform.action='page6.php'; target='_blank'; return true;">

Any ideas my friends?