Very unstable XPS8500

I have an XPS 8500 running Windows 8.1 and am having major stability issues with it. I have re-freshed the PC twice now, the first time it appeared to work for a short time but then the old problems crept back, this time they appear to have got worse. The problem started out with memory cards placed into the built in card reader not being recognised, nothing would show up in My Computer until I did a re-start, from there it has descended into frequent random crashes with a whole variety of error messages appearing, these include messages such as "Exception processing message 0xc0000006 parameters 0x00007FFFF9F264BO 0x00007FFFF9F264BO 0x00007FFFF9F264BO 0x00007FFFF9F264BO". The name at the top of the window was "taskhost.exe. Another favourite is "The instruction at 0x000000006F72A329 referenced memory at 0x000000006F72A329. The required data was not placed in memory because of an I/O error status 0x000000e". I have also had frequent problems whereby after a crash and forced re-boot the system states there is no boot device available and just opens the bios menu, when you exit out of the menu and re-start then you get back into windows again. Admittedly this last issue hasn't started to happen until after I did a bios firmware update. The really annoying part of all this is that every scan / check I do with the Dell support software or the Windows error checking software states that my computer is fine even though it is clearly not, I was thinking it was a virus but my anti virus is up to date and the scan did not find anything so I was then thinking perhaps it was hardware related like maybe a bit of ram had become corrupted and when windows tries to write to it the system crashes however I would have thought the Dell support software would have found this. The only other thing to add with all this is that the problems only appear when I am doing some heavy multi-tasking such as photoshop plus a virus scan plus converting video footage plus running internet explorer etc at the same time however when the computer was new it did all this with ease. Any ideas on how to fix this would be gratefully receive Thanks Ben