My Custom built Computer which i have used for about 7 months ocassionally does not post. Though the fans spin, hard drive spins but nothing comes up on the monitor. I have checked all the connections. When the Computer does turn on, i have stressed the CPU and the GPU using various software. I have also ran Memtest around two weeks ago with about 10 passes with failures.
You say you ran Memtest and it showed failures. That is the likely cause of your problems.
To narrow it down to the faulty component you can do a couple other tests. First, I assume you have more than one RAM module. It may be that the problem is in only one of them. Check your mobo manual and see exactly which RAM slot should be the ONLY slot used when you install only one RAM module. Then remove all the RAM and install only one module in that slot. Boot the computer and run Memtest. Repeat so that you test out the system with only one RAM module at a time, cycling through all the modules you have. That should identify the RAM module(s) (hopefully, only one) with the flaw. Replacing it should solve your problem.
When returning to the original configuration with more than one RAM module installed, make sure you install them in the correct sockets. Some boards have four slots and, to install only two modules, you MUST place them is certain slots to enable faster RAM operation. So check your manual, and, even better, note which slots are populated BEFORE you start moving them around.
It is also possible you will find no problem with any of the RAM modules this way. That might happen if the real problem was simply a poor connection in the RAM socket. In that case, replace all the RAM modules in their sockets and re-test the machine to verify the problem has been fixed.
A last thought. You've had this custom-built computer for 7 months and the problem is only recent. Sometimes a mobo BIOS mis-judges the proper voltage settings for certain components, often for RAM. You should check the specifications from the RAM makers for the proper voltage it requires, then look in the BIOS Setup menus for an indication of the actual RAM voltage setting. If they don't agree, change the RAM setting (often you have to increase it slightly) and re-test the RAM and booting performance. It's just possible your machine always was on the edge of failure for such a reason, and it only recently drifted from "on the edge" to frequent malfunction.