Try running memtest86+ again. If you continue to get the error, your RAM is bad. Then you have a couple of options:
-- you could try using the BIOS to reduce the RAM speed or increase the latencies - but I'm not sure Dell allows those changes.
-- replace the RAM.
Note: If you have 2 sticks of RAM, remove one and run memtest86+, then run it again on the other stick. That way you'll know which stick is bad.