293690 linux guru (apache related)

I have a video sharing site that has high traffic. but even if i have a strong machine (Intel(R) Core(TM)2 Quad CPU Q6600 2.40GHz, 8G RAM, 900G HDD) some days it goes OK (13000 visits per day) but every 2 or 3 days it drops to 5000. something is slowing our site. Direct admin is installed. Any job you will do will have to wait to get paid after a week after the traffic stays in 12000-13000 range (or better). I am sure it is a simple job for a professional, so keep your bids down. Please provide similar sites with high traffic the you tuned for reference. See how traffic was in the attachment.