eCommerce site ranking dropping from search engines

[SEMPro]

Hi,

I am webmaster of one eCommerce website and earlier having good traffic. Recently our portal is coming up and down again and again. Ranking position is not stable for almost keywords. Sometimes ranking dropped in few hours only and then again come back on the same position. I have loosed almost 50% of the traffic coming from search engine. Can anybody suggest me why this is happening and why a big portal ranking is not stable. I am doing proper ethical search engine optimization.

thanks,
SEMPro