checking site for demotion

how to check if somebody is doing....

[experienced]

we have a site 6 yr old, 3500 pages, pr 4, over 7.5 L page views/m... 65k unique visitors/m.. all pages indexed in google, ranked very high. got pr raise last month. since 1 Feb 2009, it is down and down every day. all rankings gone, 60% traffic reduced in just 4 days.

I believe somebody is really demoting this site somehow. i check my backlink which are fine and are from good related & decent pr and original content sites. no spam i got in google indexed so far. content is also not copied.

is there any way how to check the site if somebody is demoting the site. or what wrong is happening.

is anybody there who experiencing the similar issues...

[incrediBILL]

If you have IBLs or OBLs from bad neighborhoods they could be causing you harm.

I watched someone's site vaporize after removing link condoms from his directory due to the OBLs that went sour over time in his site.