Memory Parity Error

Biggest suspect, is memory. Use memtest on a USB or CD/DVD and let it run for about 1 day (or stop when error is already encountered). If you have 8GB of memory, and the errors happen in the 6GB region, it's the second DIMM that's defect. In case you don't know for sure, test again with only one DIMM in your mobo (starting with all DIMMs could just save time).

So I run memtest86 and it said everything is OK so what to do now

BTW I don't always get the error

It just happens sometimes on this pc

Do you have any extra memory? Sometimes memory will be malfunctioning but still pass tests. If you have two sticks of memory i would take one out and see if the issue remains. if so then that stick is that bad one.

Yes is two sticks it have

you have top be cautious that this could be the memory or the motherboard, as the system comes under load this will increase the heat within the system and things will "Expand" and move slightly. This maybe why your system is stable and only periodically fail's/blue screens. It maybe worth placing a single DIMM in the machines and run a heavy test for a number of hours then replicate this in the other DIMM slot and see if the system fails, Complete the same test with the 2nd stick of Ram and again see if the system fails. This will be a slow process to get to the root of the issue but you will rule out all items one at a time.

As you said yourself, it happens only sometimes. Therefore one run of memtest probably won't show up anything. I specifically asked you to run it one full day.

Does this motherboard actually have parity checked memory?  This was common in 1985, but is rather rare in 2015, I don't recall the last time I saw a system with parity checking. Generally, workstations have no error checking, and servers have ECC. Neither will throw parity errors.

Can you get a screenshot or photo of the error message?

do you have any BSOD's?
if yes, post the minidmp file, find it in windows\minidumps

what system is this?  some specs please : model or mobo model,
installed ram -cpu  disk drives

in such a case, i test if i can run from a bootable cd, like the ubcd mentioned above, or any live cd

Optiplex 745

running whart os ?
you did not answer my questions : " installed ram -cpu  disk drives"

**words don't cost, so i suggest to use them a bit more

Vista with 2 gigs ram

i suggest testing ram and disk first - i use the ubcd for this:
Hardware diagnostic CD    UBCD
---------------------------------------------------
go to the download page, scroll down to the mirror section, and  click on a mirror to start the download
Download the UBCD and make the cd   <<==on a WORKING PC, and boot the problem PC from it
Here 2 links, one to the general site, and a direct link to the download

since the downloaded file is an ISO file, eg ubcd527.iso - so you need to use an ISO burning tool
if you don't have that software, install cdburnerXP : http://cdburnerxp.se/

If you want also the Ram tested - run memtest86+ at least 1 full pass,  - you should have NO errors!
 
For disk Diagnostics run the disk diag for your disk brand (eg seagate diag for seagate drive)  from the HDD section -  long or advanced diag !  (runs at least for30 minutes)

http://www.ultimatebootcd.com/                        ultimate boot cd
http://www.ultimatebootcd.com/download.html             download page

**  you can make a bootable cd - or bootable usb stick

for completeness -here's how i handle disk problems : https://www.experts-exchange.com/Storage/Hard_Drives/A_3000-The-bad-hard-disk-problem.html

****you can also run from a bootable live Knoppix cd - and check if that functions ok; if yes the hardware is probably ok; if not you know where to look
http://www.knopper.net/knoppix-mirrors/index-en.html

OK, this is weird. Your machine does not use parity RAM, and does not have the hardware required to raise an NMI.  Tried the NIC driver and updating BIOS?

http://www.dell.com/support/Article/bz/en/bzdhs1/604790/EN#380parity

i was told by someone that it is most lilely the riser card failing and i should remove it and clean the contacts

what exactly is the riser card??

he said it is what the video card is seated in

this is a pic of the video card on the left and that white pci slot on the right

is the white pci slot the riser card????

no - it is a pci slot
did you run the diags i suggested ?  if not  - tell me why

Nobus the ram and HD is showing up as good