You might have seen other people having this problem:
And http://www.onemoretake.com/2009/03/27/revisited-tcpdf-variable-height-table-rows-with-multicell/ created their own class to circumvent the problem.
The fix now is as nicolaasuni says in http://sourceforge.net/projects/tcpdf/forums/forum/435311/topic/3874468?message=8669584, get the latest version of TCPDF.
Alternatively, if you can't upgrade, you can use the
nobrattribute which tells TCPDF not to break in the middle of a cell or other HTML tag. Usage:
<td>your content here</td>
Solution found (nicolaasuni again): http://sourceforge.net/projects/tcpdf/forums/forum/435311/topic/3457446?message=7762133