Why is this? HElp Google SERP

[Amenda Jessera]

One of my friend complained me the followings, I could not figure out it. Warriors please advice.

I have a site, for that I could able to rank # 2 in google for a keyword search, say A, When I try to rank top # 10 for another keyword, Say B,

the site jumps up and down for the second keyword(B).
Sometimes, it is on #10, sometimes on # 13, sometimes on # 34,
But, when I checked the ONSITE, it is fine. OFF site, I am getting 20-25 backlinks daily from authority forums and other sites.

But for that keyword, other sites(competitor sites) are stabled. So, we cannot say it is because of competition.

It is just playing somewhere....something wrong somewhere... any ideas?