Major ranking drop

Sandbox / OOP / Something else?

[elingo]

Hi - First off, thanks to the posters on here who have helped me greatly over the last year.

On to my problem - my site has dropped big-time in the G serps, and can anyone tell me what the most likely reason is? I'll summarise the site/issues:

- Site is one year old
- Homepage PR5 on recent update
- DMOZ listing for 6 months
- Yahoo listing 1 month
- Many other inbound links

- Since DMOZ listing traffic rose to around 4000 unique visitors per day
- Since March 23rd, massive drop to around 700/day
- Despite a completely unique domain name, site now ranks below 4 other pages for search on that unique word. The pages occupying the top results simply list and link to my site, and all have PR of 2 or 3. Strange.

Possible problems:

- Large site - several thousand pages
- Some generic content (but plenty of original content)
- Around 15 reciprocal links established in Jan/Feb

Questions:

- Would any of the above stand out as being a classic case for a spell in the Sandbox, even after 10 months?
- Or has a penalty maybe been applied for reciprocal linking or dup content? (although the content has been there for a year)

It ain't easy this SEO lark at times! Any hints would be much appreciated..

Cheers
Elingo