Need Help Regarding Ajax

Question

i want to use ajax for postbacking induvidual section instead of whole page.
in that section i want to draw 6 line(<td bgcolor="black"> of table), on first click i create first line but next time it gives error like " 'null' is null or not an object "

without ajax it is working, but i want to create it using ajax,
plz help
Thanks in advance

regards,
Santosh
[email protected]