Keep getting error when doing memtest

I have been having problems after an update. First problem was no dash home and upper taskbar. Could not get problem repaired run a line in termimal and dash and taskbar would be there. Then run an update before turning computer off. The next day when i turned it on all i could get was black screen with cusor in upper left corner.I was asked if i had run Memtest so i did. Running 12.04LTS. It has been doing many things but after runnig Memtest It started showing an error in Memtest. Test #7. Removed a memory card and rerun Memtest and it tested ok. After all it would do was black screen with cursor i rerun Memtest and it came up error Test #7 again. I replaced both memory cards with and run Memtest and it showed error #7 again. Do not know what to do now.