Affiliate Marketing Secrets

This is driving me absolutely nuts for the last week I’ve been fight over a keyword in google and no matter what I do, I can’t seem to get an accurate rank indication using google. Whenever I check Google.com, it keeps showing me in the No.1 spot but I keep moving up and down in the top 5.
Anyway, when I then check any other rank checker (online and offline ones) I get a totally different ranking, usually about No.11 for my main keyword.
I’ve logged out of my Google accounts, cleared all of my browser search history, cache and cookies etc. I even downloaded Internet explorer I checked with that. But each time I get these high rankings. So either I’m doing something wrong or EVERY single rank checker is??? I think it’s me but I can’t work out what it is and it’s doing my head in.. I wish Google would just leave the bloody search alone and stop personalising it!
Please help me, this is urgent because it’s an affiliate product launch and EVERY second counts. Every min I’m not No.1 I’m losing money.