What i have seen; is not the memory defect; but people have improperly installed them; one mistake is that they often touch these contacts pins with their bare hand. Long story short; just use some industrial standard cleaner; and reinstall the memory sticks back into slot; will often solve these problem.
If you in PC business long enough; you see the opportunities every time when they have brought their computer in your store. Just like any car shop; i guarantee they can point out some flaws out of your car.