VS 2008 - Multithreaded Crawler - Each Time A New Thread Accesses One Of The Lists The Content Is Changed

I have written a multithreaded crawler and the process is simply creating threads and having them access a list of urls to crawl. They then access the urls and parse the html content. All this seems to work fine. Now when I need to write to tables in a database is when I experience issues. I have 2 declared arraylists that will contain the content each thread parse. The first arraylist is simply the rss feed links and the other arraylist contains the different posts. I then use a for each loop to iterate one while sequentially incrementing the other and writing to the database. My problem is that each time a new thread accesses one of the lists the content is changed and this affects the iteration. I tried using nested loops but it did not work before and this works fine using a single thread.

Here is my

SyncLock dlock
For Each rsslink As String In finallinks
postlink = finalposts.Item(i)

[CODE]...

Finallinks and finalposts are the two arraylists. I did not include the rest of the code which shows the threads working but this is the essential part where my error occurs which is basically here postlink = finalposts.Item(i) i = i + 1

ERROR: index was out of range. Must be non-negative and less than the size of the collection. Parameter name:index

I tried copying it to a new list but dosent work.