negative seo? seo gurus help please ...

ok ... now, i am feeling like someone is fucking me right to the ass ... i have been working on new seo strategy for some time ... so far, the results are promissing ... i mean, so far some of the results ... there is a strange thing ... once i have site on top serps, it is all good ... all good, it is good for three four months or so, that is how long i am experimenting with this strategy. now, i say that only part of those things are good, because, everytime, when one guy, really only one guy, one competitor, show up on top serps, i am nowhere ... all my sites where he is not promoting similar terms, i am still stable on those top serps, but 90% of sites where he shows up, i am nowhere, i mean really nowhere, nowhere on first 10+pages ... now, how is that possible? why i am nowhere, why i am not after him, still on first page, but after him, if he is better than me? why i am nowhere? i can have those good serps for month or so, that is what i am seeing so far, stable serps, with my best know how, all is good, until the guy make his promotion, and then, i am lost, totally kicked out of google ... i know that, as every step i do with my sites is written into the info file so i know everything about my sites ... now, is that something some of you have experience with? i can pay for any help, or any study, or anything that will tell me and explain me, why my sites are lost, but only when the competitor shows up ... thank you for the reading, excuse for wall of text, but what i have on my mind, that is going out thank you in advance for any help and have luck with your legit business everyone!