posts disappearing…

Hello, I have this weird issue with the posts – for example, after google caches a post of mine, and I find that page in the search – I click thru and I get the nothing found page, like if google made that particular url up and it was never a post url of my blog, I use feedwordpress, the duplicate posts filter of feedwordpress – are there any issues know with them? My urls aren’t teh default wp urls – I set wp to put the post title in the urls so there’s lots of keywords there – I even saw a case when somebody came to a post and after that there was page not found in that url – I then dicovered that the url changed somehow and there was one word added to it and it was accesible that way, added by feedwordpress I think – as he updated the syndicated content, making a new url for that post – but why erasing the older url of that post… This happens older and newer posts alike – could that be that posts are deleted on their own? Or what else might be causing this?

Thank you in advance!