iMac 21" 2008 with 10.7.5, crashes, long boot time, no boot to login

Hi,

I got a strange behavior of my iMac. Basically the machine is working fine since 2008 ... but ... I do not remember what was first:

Sometimes in the middle of work the system does not respond to keyboard or mouse clicks whereas the mouse can still be moved. ITunes runs music for some seconds (usually internet radio). Lately my iMac has died with blanking the screen (to black) - like before the hard disk still runs. Nevertheless the only way out is to press the power button for some seconds to force poweroff. So no kernel panic or anything else. This happens really only from time to time with days or weeks in between.

Then there is the issue of booting. I remember times where the time was at maximum 30sec to start from pressing power on to login screen. In the last months my iMac began to boot slower and slower. Then it - sometimes - does not boot to login screen at all. Endlessly I see grey screens with mouse in the upper corner, then a refreshed grey screen with no mouse, then again the screen with mouse. At the end it never boots to login. Then I start the recovery tool (by holding option/alt during boot) to repair the hard disk (volume first, then access rights) with the disk utility. I see there a lot of failures which are fixed. Still it does not mean this helps. I rebooted and had the same endless boot. Then I tried to do a verbose boot to see anything and to google for - but instead of iterating through grey screens now it iterates through black screens showing the cursor 😟. The same is when having all USB and FW disconnected (but using USB keyboard plus BT Apple mouse)

I am now writing this after booting into safe mode by holding shift during boot - which seems to "work". I run the disk utility again and did the two repair options and it again found a lot of to be fixed issues.

Some time ago I had Virtual Box installed when having this problem. I deinstalled it just as a guess (in safe mode) as it uses some USB drivers. Then after restart it booted. After that I installed it again and it still worked. Nevertheless the next time I had this issue I deinstalled it completely and today it is not on my disk at all. So my previous assumption that Virtualbox disturbed my system is wrong.

My setup is:

I updated a really longer time from 10.6 to 10.7 ... today I am running 10.7.5 always using the update mechanism.

I have installed 4GB third party RAM since ever. (I already run a shorter RAM test with the iMac DVDs)

I have an external USB drive running time machine backups.

I have not really found such an issue by google. So my question is whether anyone has seen something like this and knows a cure?! if not can someone help me in finding the right console log files or keywords to search for?