Hey Hampig,
Looks like you are running a joomla site, I see you went from the initial ID way of handling articles to the SEF way and I think that is causing the problems.
First things first though like Baldea says you need to find out what errors you have, where they are coming from and why.
If you go to Webmaster Tools -> Heatlh -> Crawl Errors and click on one of the errors you can see the URL linking to your unavailable page. Also try the new link section and see if people have recently found your old site map or something and are linking from that.
I think though the problem is probably a bad extension or something similar. I ran Xenu on your web address and it returned a few 404 errors. But interestingly it also returned errors saying Forbidden Access (flooding), this looks to be a security "feature" of the SH404SEF extension used to handle your SEF requests (this is a guess whether you have it though). If too many requests come in it returns a forbidden error I guess to stop people flooding your server. This could be where Google is running up against errors. Maybe its trying to crawl your site but gets a flooding request and returns a 404 error or something similar
In order for me to run Xenu I had to drop it down to one thread so that I didnt cause too many flood errors. Eventually Xenu gave me this..
| ok | 822 URLs | 82.70% |
| connection aborted | 1 URLs | 0.10% |
| forbidden request | 30 URLs | 3.02% |
| not found | 21 URLs | 2.11% |
| skip external | 95 URLs | 9.56% |
| no info to return | 25 URLs | 2.52% |
| Total | 994 URLs | 100.00% |
As you can see 7.5% of your urls return errors when running a crawl bot against it. I bet you woudn't get this from a user, well maybe a power user opening many many tabs. You would still get the 404 errors though.
So the fact you have lots of 404 errors, Forbidden Request errors, and odd things happening with SEF i would recommend looking at your SEF Extension and see if that is causing you some problems.
Also dont be afraid to look in your server logs. Use Notepad++ to search through it. Find an example error and search for that through the log, give you lots of info (maybe too much though if you are just starting out).
Adam
edit:
you are also running JCH_Optimise to group your CSS and Javascript files. This can cause 500 errors. Try clearing the cache and run xenu against it.